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I have figured out how to add a cross over words with tcolorbox

documentclassarticleusepackagexcolor usepackage[most]tcolorbox

begindocument

Of course, presenting this solution would be cheating. And, no, it won't work in all cases, due to strict limitations on the pairs 
tcbox[tcbox raise base,
breakable,nobeforeafter, enhanced jigsaw, opacityback=0, sharp corners, parbox=false, boxrule=0pt, top=0pt,bottom=0pt,left=0pt,right=0pt, boxsep=0pt, frame hidden, parbox=false, 
 finish=draw[thick,red] (frame.south west)--(frame.north east);draw[thick,red] (frame.south east)--(frame.north west);]
 accepted
 
 by textbackslashline. Your professor surely knows that TeX doesn't draw oblique lines. With the standard package pict2e it's easier and it will work in any case.

enddocument

Here I cross out word accept. However, I want the cross to stretch out a bit, say 1cm, see my illustration.

How can I achieve that?

You can shorten your lines.

documentclassarticleusepackagexcolor usepackage[most]tcolorbox

tikzsetcrossout/.style=thick,red,shorten >=-.5cm,shorten <=-.5cm

begindocument

Of course, presenting this solution would be cheating. And, no, it won't work in all cases, due to strict limitations on the pairs 
tcbox[tcbox raise base,
breakable,nobeforeafter, enhanced jigsaw, opacityback=0, sharp corners, parbox=false, boxrule=0pt, top=0pt,bottom=0pt,left=0pt,right=0pt, boxsep=0pt, frame hidden, parbox=false, 
 finish=draw[crossout] (frame.south west)--(frame.north east);draw[crossout] (frame.south east)--(frame.north west);]
 accepted
 
 by textbackslashline. Your professor surely knows that TeX doesn't draw oblique lines. With the standard package pict2e it's easier and it will work in any case.

enddocument

An alternative TikZ solution (less elegant than TeXnician's one).

With overlay you can write over something without occupy space.

I've used shift to enlarge the lines (you can use shorten also here, but I haven't used it because TeXnician already did it, hence I would like to show an alternative).

To align TikZ nodes to normal text, see here: TikZ node in normal text.

documentclassarticleusepackagexcolor usepackagetikz
newcommandmycrossed[1]%
 tikz[remember picture, baseline=(A.base)]
 node[inner sep=0pt](A)#1;
 %
 tikz[overlay, remember picture]
 draw[red, very thick] ([shift=(-.5,.2)]A.north west) -- ([shift=(.5,-.2)]A.south east);
 draw[red, very thick] ([shift=(-.5,-.2)]A.south west) -- ([shift=(.5,.2)]A.north east);
 %
 
begindocument

Of course, presenting this solution would be cheating. And, no, it won't work in all cases, due to strict limitations on the pairs 
mycrossedaccepted
 by textbackslashline. Your professor surely knows that TeX doesn't draw oblique lines. With the standard package pict2e it's easier and it will work in any case.
enddocument