Duke, dissolute man, Sir Francis (5)
I believe the answer is:
drake
'sir francis' is the definition.
(I've seen this in another clue)
'duke dissolute man' is the wordplay.
'duke' becomes 'd' (abbreviation).
'dissolute man' becomes 'rake' (I've seen this in another clue).
'd'+'rake'='DRAKE'
(Other definitions for drake that I've seen before include "sea captain" , "Male water-bird" , "Captain of the Golden Hind, d. 1596" , "might be after a duck?" , "flyer" .)