During conflict, duke reveals secret (6)
I believe the answer is:
inward
'secret' is the definition.
(thesaurus)
'during conflict duke' is the wordplay.
'during' becomes 'in' (I've seen this in other clues).
'conflict' becomes 'war' (war is a kind of conflict).
'duke' becomes 'd'.
'in'+'war'+'d'='INWARD'
'reveals' acts as a link.
(Other definitions for inward that I've seen before include "In the mind or spirit, not outside" , "Existing in the mind - to the centre" , "Formerly private" , "Towards the centre" , "Situated on the inside" .)