Students have to copy part of jacket (5)
I believe the answer is:
lapel
'part of jacket' is the definition.
(I've seen this before)
'students have to copy' is the wordplay.
'students' becomes 'LL' (two abbreviations for learner).
'have' indicates putting letters inside.
'to copy' becomes 'ape' (ape can mean to copy or imitate).
'll' enclosing 'ape' is 'LAPEL'.
(Other definitions for lapel that I've seen before include "Lower extension of collar on jacket or coat" , "Continuation of a coat collar" , "Part of a folded back collar" , "Revere on a coat" , "Lower part of collar on coat" .)