What Batman puts on Robin's head in jest (5)
I believe the answer is:
caper
'jest' is the definition.
'what batman puts on robin's head' is the wordplay.
'what batman puts on' becomes 'cape'.
'head' says to take the initial letters.
The initial letter of 'robins' is 'r'.
'cape'+'r'='CAPER'
'in' is the link.
(Other definitions for caper that I've seen before include "Dance playfully" , "Ridiculous escapade" , "jump around?" , "practical joke" , "Pickled flower bud or prank" .)