Choose detectives to probe curtailed action (6)
I believe the answer is:
decide
'choose' is the definition.
(thesaurus)
'detectives to probe curtailed action' is the wordplay.
'detectives' becomes 'cid'.
'to probe' means one lot of letters goes inside another.
'curtailed' means to remove the last letter.
'action' becomes 'deed' (deed is a kind of action).
'deed' with its final letter taken away is 'dee'.
'cid' inserted into 'dee' is 'DECIDE'.
(Other definitions for decide that I've seen before include "Make up one's mind" , "Make a judgment" , "Judge" , "Come to a resolution" , "Reach a judgment" .)