Small sea mollusc in sink (7)
I believe the answer is:
scuttle
'sink' is the definition.
(to deliberately sink a ship)
'small sea mollusc' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sea mollusc' becomes 'cuttle' (cuttlefish).
's'+'cuttle'='SCUTTLE'
'in' acts as a link.
(Other definitions for scuttle that I've seen before include "container carrying coal?" , "Container with lid" , "Scurry - sink - coal container" , "receptacle for nuts" , "Destroy - dash - coal container" .)