Civvies (5)
I believe the answer is:
mufti
'civvies' is the definition.
(both can mean non-uniform)
This is all the clue.
(Other definitions for mufti that I've seen before include "Plain clothes worn by soldier" , "Civvy dress" , "Civilian dress, in army terms" , "Muslim religious jurist" , "Plain clothes - Muslim legal expert" .)