Some mackerel is hard to enjoy (6)
I believe the answer is:
relish
'to enjoy' is the definition.
('relish' can be a synonym of 'enjoy')
'some mackerel is hard' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'RELISH' is hidden in the letters of 'mackerel is hard'.
(Other definitions for relish that I've seen before include "Savour; an appetiser" , "savoury stuff" , "Pleasure - dressing" , "Great delight" , "Enjoy greatly - such sauce!" .)