Creature might ultimately do leap when developed (7)
I believe the answer is:
tadpole
'creature' is the definition.
(tadpole is a kind of animal)
'might ultimately do leap when developed' is the wordplay.
'ultimately' indicates one should take the final letters.
'when developed' indicates an anagram.
The final letter of 'might' is 't'.
'do'+'leap'='doleap'
'doleap' anagrammed gives 'adpole'.
't'+'adpole'='TADPOLE'
(Other definitions for tadpole that I've seen before include "Legless creature" , "Embryonic frog" , "Pre-frog form" , "young swimmer" , "youngster" .)