A halibut cooked as is customary (8)
I believe the answer is:
habitual
'is customary' is the definition.
('habitual' can be a synonym of 'customary')
'a halibut cooked' is the wordplay.
'cooked' indicates anagramming the letters.
'a'+'halibut'='ahalibut'
'ahalibut' with letters rearranged gives 'HABITUAL'.
'as' is the link.
(Other definitions for habitual that I've seen before include "Regular, usual" , "Done constantly or regularly (8)" , "set" , "Common" , "Customary, regular" .)