Duke intervening in a falling-out at sea (6)
I believe the answer is:
adrift
'at sea' is the definition.
(drifting at sea)
'duke intervening in a falling-out' is the wordplay.
'duke' becomes 'd' (abbreviation).
'intervening in' is an insertion indicator.
'falling out' becomes 'rift' (I've seen this in another clue).
'a'+'rift'='arift'
'd' put into 'arift' is 'ADRIFT'.
(Other definitions for adrift that I've seen before include "(Of ship) out of control" , "Unsettled or unanchored" , "Aimlessly floating" , "without guidance?" , "Going out of control" .)