Some desultory setback turned to advantage (4)
I believe the answer is:
used
'some desultory setback turned to advantage' is the definition.
I can't tell whether this defines the answer.
'turned to advantage' is the wordplay.
'turned' becomes 'd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'advantage' becomes 'use' (I've seen this before).
'd' after 'use' is 'USED'.
Can you help me to learn more?
(Other definitions for used that I've seen before include "Sued (anag.)" , "Sued, having been employed" , "Second-hand; consumed" , "was milking" , "Second-hand, not new" .)