Applaud a pupil for coming in top (4)
I believe the answer is:
clap
'applaud' is the definition.
(synonyms)
'a pupil for coming in top' is the wordplay.
'a pupil' becomes 'L' (as in a learner driver with L-plates on their car).
'for coming in' means one lot of letters goes inside another.
'top' becomes 'cap' (both can mean to beat or outdo).
'l' going inside 'cap' is 'CLAP'.
(Other definitions for clap that I've seen before include "express approval" , "Sudden loud noise (of thunder?)" , "Audibly show approval" , "Make sound of applause" , "Applaud manually" .)