Marina utilised for keeping molluscs (7)
I believe the answer is:
nautili
'molluscs' is the definition.
(nautilus is a kind of mollusc)
'marina utilised for keeping' is the wordplay.
'keeping' indicates a hidden word.
'NAUTILI' can be found hidden inside 'marina utilised for'.
(Other definitions for nautili that I've seen before include "creatures in the sea" , "marine life" , "Cephalopod molluscs" .)