Detectives to be involved in a French exploit? That's doubtful (9)
I believe the answer is:
undecided
'doubtful' is the definition.
(thesaurus)
'detectives to be involved in a french exploit?' is the wordplay.
'detectives' becomes 'cid'.
'to be involved in' indicates putting letters inside.
'a french' becomes 'un' ('a' in French).
'exploit?' becomes 'deed' ('deed' can be a synonym of 'exploit').
'un'+'deed'='undeed'
'cid' inserted into 'undeed' is 'UNDECIDED'.
'that's' acts as a link.
(Other definitions for undecided that I've seen before include "With mind not yet made up" , "Dithering" , "Not settled" , "Not yet having made a commitment" , "Still up in the air?" .)