Student wearing Australian mate's gear (7)
I believe the answer is:
'gear' is the definition.
(I've seen this before)
'student wearing australian mate's' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'wearing' means one lot of letters goes inside another.
'australian mate' becomes 'cobber' (AUS slang for a friend).
'l' placed inside 'cobber' is 'CLOBBER'.
(Other definitions for clobber that I've seen before include "Lick" , "Tackle" , "stuff" , "Repeatedly hit" , "Hit; gear (sl.)" .)