Like mackerel, really good (4)
I believe the answer is:
holy
'good' is the definition.
(thesaurus)
'like mackerel really' is the wordplay.
I cannot really see how this works, but
'like' could be 'ly' (similar in meaning) and 'ly' is found within the answer.
The remaining letters 'ho' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for holy that I've seen before include "Sanctified" , "Sacred" , "Sacrosanct" , "perfect, morally and spiritually" , "Charlemagne was the first" .)