Animal held in prison pounced on leaders (5)
I believe the answer is:
hippo
'animal' is the definition.
(I've seen this before)
'held in prison pounced on leaders' is the wordplay.
'leaders' suggests taking the first letters.
The first letters of 'held in prison pounced on' is 'HIPPO'.
(Other definitions for hippo that I've seen before include "Semiaquatic animal" , "Great beast" , "large African creature" , "Large African beast (abbr)" , "African wallower (abbr)" .)