Young frog in Po Delta, developing (7)
I believe the answer is:
tadpole
'young frog' is the definition.
(I've seen this before)
'po delta developing' is the wordplay.
'developing' indicates anagramming the letters.
'po'+'delta'='podelta'
'podelta' with letters rearranged gives 'TADPOLE'.
'in' acts as a link.
(Other definitions for tadpole that I've seen before include "Embryonic frog" , "Frog in formation" , "Amphibian larva" , "Pre-frog form" , "developed from spawn" .)