Dons return with unknown assembly (5)
I believe the answer is:
synod
'assembly' is the definition.
(I've seen this before)
'dons return with unknown' is the wordplay.
'return' says the letters should be written backwards.
'with' is an insertion indicator.
'unknown' becomes 'y' (algebra).
'dons' in reverse letter order is 'snod'.
'snod' enclosing 'y' is 'SYNOD'.
(Other definitions for synod that I've seen before include "Council of clergy to discuss policy" , "Assembly of the clergy" , "Ecclesiastic council" , "Forum for discussing" , "Ecumenical council" .)