Guarantor undoubtedly voided treaty (6)
I believe the answer is:
surety
'guarantor' is the definition.
(I know that guarantor can be written as surety)
'undoubtedly voided treaty' is the wordplay.
'undoubtedly' becomes 'sure'.
'voided' suggests removing the centre (void can mean to empty out).
'treaty' with its middle taken out is 'ty'.
'sure'+'ty'='SURETY'
(Other definitions for surety that I've seen before include "Legal safeguard against loss" , "Semitic language" , "Safeguard given against financial loss" , "Security - guarantee" , "Warranty, guarantee" .)