Top deal ruined by youngster (7)
I believe the answer is:
tadpole
'youngster' is the definition.
(I've seen this before)
'top deal ruined' is the wordplay.
'ruined' indicates an anagram.
'top'+'deal'='topdeal'
'topdeal' with letters rearranged gives 'TADPOLE'.
'by' acts as a link.
(Other definitions for tadpole that I've seen before include "wriggler?" , "little swimmer" , "Embryonic frog" , "Young toad" , "Larval frog" .)