Little swimmer prepared to plead (7)
I believe the answer is:
tadpole
'little swimmer' is the definition.
(I've seen this before)
'prepared to plead' is the wordplay.
'prepared' is an anagram indicator.
'to'+'plead'='toplead'
'toplead' is an anagram of 'TADPOLE'.
(Other definitions for tadpole that I've seen before include "Old tape (anag.)" , "Larval toad" , "young swimmer" , "Potential frog" , "wriggler?" .)