Hippopotamus I captured holding rock, perhaps (5)
I believe the answer is:
music
'rock perhaps' is the definition.
(rock is a kind of music)
'hippopotamus i captured holding' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'MUSIC' is hidden within 'hippopotamus i captured'.
(Other definitions for music that I've seen before include "graces?" , "Airs" , "A score represents it" , "Staff will support this" , "Crafted sound" .)