Civilian clothes (5)
I believe the answer is:
'civilian clothes' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mufti that I've seen before include "Civilian dress, worn by an off-duty soldier, say" , "Non-military clothing" , "Civvy dress" , "Soldier's civilian clothes" , "Muslim religious jurist" .)