Swimmer dislodges top from sink (6)
I believe the answer is:
cuttle
'swimmer' is the definition.
(cuttlefish, sea animal)
'dislodges top from sink' is the wordplay.
'dislodges top from' means to remove the first letter.
'sink' becomes 'scuttle' (to deliberately sink a ship).
'scuttle' with its first letter taken off is 'CUTTLE'.
(Other definitions for cuttle that I've seen before include "Species of mollusc with shield-shaped body" , "Squid-like mollusc" .)