Young swimmer first when racing, bit ahead (7)
I believe the answer is:
tadpole
'young swimmer' is the definition.
(young frog)
'first when racing bit ahead' is the wordplay.
'first when racing' becomes 'pole' (in pole position).
'bit' becomes 'tad' (both can mean a small amount).
'ahead' says to put letters next to each other (some letters go in front of others).
'pole' put after 'tad' is 'TADPOLE'.
(Other definitions for tadpole that I've seen before include "Young toad" , "Baby amphibian" , "Young amphibian" , "little swimmer" , "Immature toad or frog" .)