Racket sport's ultimate force (4)
I believe the answer is:
dint
'force' is the definition.
(I've seen this before)
'racket sport's ultimate' is the wordplay.
'racket' becomes 'din' (I've seen this before).
'ultimate' says to take the final letters.
The final letter of 'sport' is 't'.
'din'+'t'='DINT'
(Other definitions for dint that I've seen before include "(By) means (of)" , "Impression or hollow in a surface" , "Force" .)