You must tuck into fish spread (4)
I believe the answer is:
daub
'spread' is the definition.
(thesaurus)
'you must tuck into fish' is the wordplay.
'you' becomes 'u' (commonly used in online communication).
'must tuck into' is an insertion indicator.
'fish' becomes 'dab' (dab is a type of fish).
'u' inserted inside 'dab' is 'DAUB'.
(Other definitions for daub that I've seen before include "Smear; plaster" , "Carelessly spread (paint)" , "Plaster; paint crudely" , "Amateurish painting" , "Do graffiti" .)