Students have to copy part of a jacket (5)
I believe the answer is:
lapel
'part of a jacket' is the definition.
(I've seen this before)
'students have to copy' is the wordplay.
'students' becomes 'LL' (an abbreviation for learner driver written twice).
'have' means one lot of letters goes inside another.
'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 "Feature of a jacket" , "Turnover at the front of a coat" , "Revere on a coat" , "Lower extension of collar on jacket or coat" , "top of coat" .)