Part of unit fuming to be sent back in plain clothes (5)
I believe the answer is:
mufti
'plain clothes' is the definition.
(I've seen this before)
'part of unit fuming to be sent back' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'to be sent back' shows that the letters should be reversed in order.
'itfum' can be found hidden inside 'unit fuming'.
'itfum' reversed gives 'MUFTI'.
'in' is the link.
(Other definitions for mufti that I've seen before include "Soldier's civilian clothes" , "Civilian clothes worn by service personnel" , "Plain clothes worn by soldier" , "Civilian gear" , "No official dress" .)