Study hard when fool finished (3,2)
I believe the answer is:
mug up
'study hard' is the definition.
(I've seen this before)
'fool finished' is the wordplay.
'fool' becomes 'mug' (synonyms).
'finished' becomes 'up' (eg 'time's up' means the time is finished).
'mug'+'up'='MUG UP'
'when' acts as a link.
(Other definitions for mug up that I've seen before include "Quickly revise" , "Study quickly (for, eg, an exam)" , "Study hard" , "Revise for an exam" , "Learn subject rapidly" .)