Taking part in Monica Seles's action (4)
I believe the answer is:
case
'action' is the definition.
(synonyms)
'taking part in monica seles's' is the wordplay.
'taking part in' indicates a hidden word.
'CASE' is hidden within 'monica seles'.
(Other definitions for case that I've seen before include "Keep careful watch on" , "proceedings" , "Court evidence" , "Check out" , "Container and court proceeding" .)