Sign supporting mostly inauspicious opponents (11)
I believe the answer is:
adversaries
'opponents' is the definition.
('adversary' can be a synonym of 'opponent')
'sign supporting mostly inauspicious' is the wordplay.
'sign' becomes 'aries' (star sign**).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'mostly' means to remove the last letter.
'inauspicious' becomes 'adverse' ('adverse' can be a synonym of 'inauspicious').
'adverse' with its final letter removed is 'advers'.
'aries' after 'advers' is 'ADVERSARIES'.
(Another definition for adversaries that I've seen is " Enemies".)