Creature prepared to leap across delta (7)
I believe the answer is:
tadpole
'creature' is the definition.
(tadpole is a kind of animal)
'prepared to leap across delta' is the wordplay.
'prepared' indicates anagramming the letters.
'across' is an insertion indicator.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'to'+'leap'='toleap'
'toleap' anagrammed gives 'tapole'.
'tapole' going around 'd' is 'TADPOLE'.
(Other definitions for tadpole that I've seen before include "Larval toad" , "Embryonic frog" , "Larval form" , "Baby frog" , "Immature toad or frog" .)