Small mollusc in sink (7)
I believe the answer is:
scuttle
'sink' is the definition.
(I've seen this before)
'small mollusc' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mollusc' becomes 'cuttle' (cuttle is a kind of mollusc).
's'+'cuttle'='SCUTTLE'
'in' is the link.
(Other definitions for scuttle that I've seen before include "Coal bucket" , "dish?" , "Sink one's ship deliberately" , "Container with lid" , "Rush" .)