Deal with unknown contract (6)
I believe the answer is:
treaty
'contract' is the definition.
(I've seen this before)
'deal with unknown' is the wordplay.
'deal with' becomes 'treat' ('treat' can be a synonym of 'deal with').
'unknown' becomes 'y' (algebra).
'treat'+'y'='TREATY'
(Other definitions for treaty that I've seen before include "settlement" , "Formal agreement between nations" , "Pact, accord between states" , "Formal agreement between states" , "Formal pact" .)