Dons return with unknown assembly (5)
I believe the answer is:
synod
'assembly' is the definition.
(thesaurus)
'dons return with unknown' is the wordplay.
'return' is a reversal indicator.
'with' indicates putting letters inside.
'unknown' becomes 'y' (algebra).
'dons' written backwards gives '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" .)