Muslim legal expert in plain clothes (5)
I believe the answer is:
mufti
I believe this clue is a double definition.
'muslim legal expert' is the first definition.
(I've seen this before)
'plain clothes' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for mufti that I've seen before include "Civilian dress, in army terms" , "Plain clothes worn by soldier" , "Plain clothes - Muslim legal expert" , "Soldier's civilian clothes" , "Civvy dress" .)