Students have to copy collar part (5)
I believe the answer is:
lapel
'collar part' is the definition.
'lapel' can be an answer for 'part' (I have seen 'Part of jacket' mean 'lapel' so perhaps 'part' could also mean 'lapel'). I am unsure of the 'collar' bit.
'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' going around 'ape' is 'LAPEL'.
Can you help me to learn more?
(Other definitions for lapel that I've seen before include "Part of a folded back collar" , "Turnover at the front of a coat" , "Folded flap at front of jacket or coat" , "Revere on a coat" , "Lower extension of collar on jacket or coat" .)