Students about to copy part of jacket (5)
I believe the answer is:
lapel
'part of jacket' is the definition.
(I've seen this before)
'students about to copy' is the wordplay.
'students' becomes 'll'.
'about' indicates putting letters inside.
'to copy' becomes 'ape' ('ape' can be a synonym of 'copy').
'll' enclosing 'ape' is 'LAPEL'.
(Other definitions for lapel that I've seen before include "Lower part of collar on coat" , "Feature of a jacket" , "Extended part of a collar" , "Turnover at the front of a coat" , "top of coat" .)