Challenging former deputy (8)
I believe the answer is:
exacting
'challenging' is the definition.
(I've seen this before)
'former deputy' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'deputy' becomes 'acting' (I've seen this in other clues).
'ex'+'acting'='EXACTING'
(Other definitions for exacting that I've seen before include "Meticulous" , "Harsh" , "Demanding great skill" , "Challenging" , "Requiring great effort - sounds precise" .)