Mollusc’s not a water pig (6)
I believe the answer is:
quahog
'mollusc's' is the definition.
(quahog is a kind of mollusc)
'not a water pig' is the wordplay.
'not' indicates named letters should be taken away.
'water' becomes 'aqua' (I've seen this before).
'pig' becomes 'hog' (synonyms).
'aqua' with 'a' taken out is 'qua'.
'qua'+'hog'='QUAHOG'
(Other definitions for quahog that I've seen before include "North American edible clam" , "Round clam" .)