Deal with unknown alliance (6)
I believe the answer is:
treaty
'alliance' is the definition.
(thesaurus)
'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 "Covenant" , "Pact, accord between states" , "something of Rome or Versailles?" , "convention?" , "'Pact, accord (6)'" .)