Unexceptional Russian agreement's rejected to achieve parity (8)
I believe the answer is:
adequate
'unexceptional' is the definition.
(thesaurus)
'russian agreement's rejected to achieve parity' is the wordplay.
'russian agreement' becomes 'da'.
'rejected' is a reversal indicator.
'to achieve parity' becomes 'equate'.
'da' reversed gives 'ad'.
'ad'+'equate'='ADEQUATE'
(Other definitions for adequate that I've seen before include "Passable; enough" , "Barely sufficient" , "plenty" , "Enough to meet a purpose" , "Fair to middling" .)