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' says the answer is hidden in the clue.
'RELISH' can be found hidden inside '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" .)