The flavour of mackerel is horrible (6)
I believe the answer is:
relish
'the flavour' is the definition.
('relish' can be a synonym of 'flavour')
'of mackerel is horrible' is the wordplay.
'of' indicates a hidden word.
'RELISH' is hidden in the letters of 'mackerel is horrible'.
(Other definitions for relish that I've seen before include "savoury preserve" , "sauce required?" , "'Take pleasure in, savour (6)'" , "smack" , "Enjoy the highly seasoned sauce" .)