Heather; marine fish (4)
I believe the answer is:
ling
This clue is a double definition.
'heather' is the first definition.
(synonyms)
'marine fish' is the second definition.
(I've seen this in another clue)
(Other definitions for ling that I've seen before include "Common heather" , "Burbot" , "Slender food fish" , "Heather - fish" , "Cod-like fish" .)