Vengeful Greek soon to be leading troops (7)
I believe the answer is:
electra
'vengeful greek' is the definition.
(figure of Greek myth)
'soon to be leading troops' is the wordplay.
'soon to be' becomes 'elect' (eg the president elect is soon to be president).
'leading' is a charade indicator (letters next to each other).
'troops' becomes 'ra' (Royal Artillery).
'elect'+'ra'='ELECTRA'
(Other definitions for electra that I've seen before include "Mythical woman with a complex?" , "Daughter of Agamemnon and Clytemnestra (who has a complex?)" , "female in tragedy" , "Agamemnon's daughter" , "Treacle (anag) - daughter of Agamemnon" .)