Firm runs through claim in court (4)
I believe the answer is:
sure
'firm' is the definition.
(I've seen this in other clues)
'runs through claim in court' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'through' indicates putting letters inside (inserted letters go through the word).
'claim in court' becomes 'sue' (as in suing in court).
'r' placed within 'sue' is 'SURE'.
(Other definitions for sure that I've seen before include "Certain to be a ruse" , "Positive" , "See 12" , "of course" , "Agreed" .)