A university eleven complain bitterly over unknown supporter (9)
I believe the answer is:
auxiliary
'supporter' is the definition.
(I know that auxiliary is a type of supporter)
'a university eleven complain bitterly over unknown' is the wordplay.
'university' becomes 'U' (abbreviation for university).
'eleven' becomes 'xi' (Roman numerals).
'complain bitterly' becomes 'rail' (I've seen this before).
'over' shows that the letters should be reversed in order.
'unknown' becomes 'y' (algebra).
'rail' in reverse letter order is 'liar'.
'a'+'u'+'xi'+'liar'+'y'='AUXILIARY'
(Other definitions for auxiliary that I've seen before include "Providing additional support" , "Acting as subsidiary or support, for an army perhaps" , "Secondary" , "Helping or supporting" , "Subordinate" .)