Head removed from bygone sea creature (5)
I believe the answer is:
ormer
'sea creature' is the definition.
(I've seen this before)
'head removed from bygone' is the wordplay.
'head removed from' means to remove the first letter.
'bygone' becomes 'former' (as in bygone times).
'former' with its first letter taken away is 'ORMER'.
(Other definitions for ormer that I've seen before include "Edible abalone; rhymes with projecting window" , "Sea-ear, an abalone found near the Channel Is" , "sea creature" , "Shellfish" , "Mollusc" .)