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