French nobleman in action worked out (7)
I believe the answer is:
deduced
'worked out' is the definition.
(I've seen this before)
'french nobleman in action' is the wordplay.
'french nobleman' becomes 'duc' ('duke' in French).
'in' is an insertion indicator.
'action' becomes 'deed' (deed is a kind of action).
'duc' put inside 'deed' is 'DEDUCED'.
(Other definitions for deduced that I've seen before include "Inferred" , "Gathered" , "Came to logical conclusion like Sherlock" , "Worked out by reasoning" , "Arrived at by reasoning" .)