Handle termination of voluntary agreement (6)
I believe the answer is:
treaty
'agreement' is the definition.
(I know that agreement can be written as treaty)
'handle termination of voluntary' is the wordplay.
'handle' becomes 'treat' (synonyms).
'termination of' says to take the final letters (back).
The final letter of 'voluntary' is 'y'.
'treat'+'y'='TREATY'
(Other definitions for treaty that I've seen before include "Pact or agreement between nations" , "Written agreement between two states" , "'Pact, accord (6)'" , "Political or private agreement" , "Formal agreement between governments" .)