It is customary to cook a halibut (8)
I believe the answer is:
habitual
'it is customary' is the definition.
(synonyms)
'cook a halibut' is the wordplay.
'cook' indicates an anagram.
'a'+'halibut'='ahalibut'
'ahalibut' anagrammed gives 'HABITUAL'.
'to' is the link.
(Other definitions for habitual that I've seen before include "Chronic" , "Traditional" , "Done regularly and repeatedly" , "Customary, regular" , "Commonly practised" .)