Portion of mackerel is hard to enjoy (6)
I believe the answer is:
relish
'to enjoy' is the definition.
('relish' can be a synonym of 'enjoy')
'portion of mackerel is hard' is the wordplay.
'portion of' says the answer is hidden in the clue.
'RELISH' is hidden in the letters of 'mackerel is hard'.
(Other definitions for relish that I've seen before include "Salsa-like sauce" , "Much enjoy" , "Great delight" , "appetizer?" , "Savour; chutney" .)