Civvies leaving unit fuming on the way back? (5)
I believe the answer is:
mufti
'civvies' is the definition.
(both can mean non-uniform)
'leaving unit fuming on the way back?' is the wordplay.
'leaving' indicates a hidden word (the answer must leave the confines of the clue).
'on the way back?' says the letters should be written in reverse (I've seen 'on the way back' mean this).
'itfum' is hidden within 'unit fuming'.
'itfum' in reverse letter order is 'MUFTI'.
(Other definitions for mufti that I've seen before include "Soldier's civilian clothes" , "Civilian dress, in army terms" , "For soldiers, civilian dress" , "Plain clothes - Muslim law expert" , "Plain clothes worn by soldier" .)