Young creature to leap explosively across delta (7)
I believe the answer is:
tadpole
'young creature' is the definition.
'to leap explosively across delta' is the wordplay.
'explosively' indicates an anagram.
'across' is an insertion indicator.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'to'+'leap'='toleap'
'toleap' anagrammed gives 'tapole'.
'tapole' placed around 'd' is 'TADPOLE'.
(Other definitions for tadpole that I've seen before include "wriggler?" , "Amphibian larva" , "immature sea squirt" , "frog may spring from" , "young swimmer" .)