Expect witness to follow warning (7)
I believe the answer is:
foresee
'expect' is the definition.
(thesaurus)
'witness to follow warning' is the wordplay.
'witness' becomes 'see' (I've seen this before).
'to follow' is a charade indicator (letters next to each other).
'warning' becomes 'fore' (I've seen this in other clues).
'see' after 'fore' is 'FORESEE'.
(Other definitions for foresee that I've seen before include "To know beforehand" , "Predict" , "Imagine" , "Realise or be aware of in advance" , "Expect" .)