This ultimately destined to leap, when developed (7)
I believe the answer is:
tadpole
'this' is the definition.
The definition suggests a singular noun which matches the answer.
'ultimately destined to leap when developed' is the wordplay.
'ultimately' suggests the final letters.
'when developed' indicates an anagram.
The final letter of 'destined' is 'd'.
'd'+'to'+'leap'='dtoleap'
'dtoleap' anagrammed gives 'TADPOLE'.
(Other definitions for tadpole that I've seen before include "Little creature in water" , "frog may spring from" , "wriggler?" , "little swimmer" , "Frog in the making" .)