It's customary for a halibut to be stewed (8)
I believe the answer is:
habitual
'it's customary' is the definition.
('habitual' can be a synonym of 'customary')
'a halibut to be stewed' is the wordplay.
'to be stewed' indicates an anagram.
'a'+'halibut'='ahalibut'
'ahalibut' is an anagram of 'HABITUAL'.
'for' is the link.
(Other definitions for habitual that I've seen before include "Done constantly by a halibut" , "Done regularly and repeatedly" , "Customary, regular" , "Done constantly or regularly (8)" , "Common" .)