Don's upset with unknown council (5)
I believe the answer is:
synod
'council' is the definition.
(I know that synod is a type of council)
'don's upset with unknown' is the wordplay.
'upset with' is an anagram indicator.
'unknown' becomes 'y' (algebra).
'dons' anagrammed with 'y' gives 'SYNOD'.
(Other definitions for synod that I've seen before include "Ecumenical council" , "Church assembly" , "Clerical assembly" , "Forum for discussing" , "cloth gathered here" .)