Difficult former deputy (8)
I believe the answer is:
exacting
'difficult' is the definition.
(I've seen this before)
'former deputy' is the wordplay.
'former' becomes 'ex'.
'deputy' becomes 'acting' (I've seen this before).
'ex'+'acting'='EXACTING'
(Other definitions for exacting that I've seen before include "Harsh" , "Insisting upon" , "Greatly demanding" , "Challenging" , "Testing endurance or skill" .)