This do leap: might ultimately when developed (7)
I believe the answer is:
tadpole
'this' is the definition.
The definition suggests a singular noun which matches the answer.
'do leap might ultimately when developed' is the wordplay.
'ultimately' suggests the final letters.
'when developed' indicates an anagram.
The last letter of 'might' is 't'.
'do'+'leap'+'t'='doleapt'
'doleapt' with letters rearranged gives 'TADPOLE'.
Can you help me to learn more?
(Other definitions for tadpole that I've seen before include "little swimmer" , "frog may spring from" , "Larval form" , "Little creature in water" , "Baby amphibian" .)