A small role took effect on duty (3,4)
I believe the answer is:
bit part
'a small role' is the definition.
(I've seen this in another clue)
'took effect on duty' is the wordplay.
'took effect' becomes 'bit' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'duty' becomes 'part' (part is a kind of duty).
'bit'+'part'='BIT PART'
(Other definitions for bit part that I've seen before include "extra work, perhaps" , "Very minor role" , "Tiny or non-speaking role in play" , "Small acting role" , "Tiny or non-speaking dramatic role" .)