Fish; heather (4)
I believe the answer is:
ling
I believe this clue is a double definition.
'fish' is the first definition.
(ling is a kind of fish)
'heather' is the second definition.
(synonyms)
(Other definitions for ling that I've seen before include "Swimmer" , "Fish of the cod family" , "Heather - fish" , "Burbot" , "Cod-like fish" .)