Young swimmer organising top deal (7)
I believe the answer is:
tadpole
'young swimmer' is the definition.
(young frog)
'organising top deal' is the wordplay.
'organising' indicates anagramming the letters.
'top'+'deal'='topdeal'
'topdeal' with letters rearranged gives 'TADPOLE'.
(Other definitions for tadpole that I've seen before include "Baby frog" , "Potential frog" , "Amphibian larva" , "youngster" , "Legless creature" .)