Some sit fuming about legal expert (5)
I believe the answer is:
mufti
'legal expert' is the definition.
(I know that mufti is a type of legal expert)
'some sit fuming about' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'about' says the letters should be written in reverse.
'itfum' is hidden within 'sit fuming'.
'itfum' written backwards gives 'MUFTI'.
(Other definitions for mufti that I've seen before include "Civilian dress (for serviceman)" , "Civvies; Muslim legal expert" , "Civilian dress, in army terms" , "Non-military clothing" , "Soldier's civilian clothes" .)