Immature swimmer bit staff (7)
I believe the answer is:
tadpole
'immature swimmer' is the definition.
(immature amphibian which will swim)
'bit staff' is the wordplay.
'bit' becomes 'tad' (both can mean a small amount).
'staff' becomes 'pole' (I've seen this before).
'tad'+'pole'='TADPOLE'
(Other definitions for tadpole that I've seen before include "frog may spring from" , "Young amphibian" , "being hatched in a puddle?" , "Baby frog" , "Future frog maybe" .)