Heartless seabirds turned up mollusc (4)
I believe the answer is:
slug
'mollusc' is the definition.
(slug is a kind of mollusc)
'heartless seabirds turned up' is the wordplay.
'heartless' means to remove the middle letters.
'seabirds' becomes 'gulls' (gull is a kind of seabird).
'turned up' is a reversal indicator.
'gulls' with its middle removed is 'guls'.
'guls' back-to-front is 'SLUG'.
(Other definitions for slug that I've seen before include "Heavy blow for snail lacking shell" , "draught" , "Wallop" , "Bullet; swig" , "gulp!" .)