Agreement was sentimental to some extent (6)
I believe the answer is:
'agreement' is the definition.
(I know that assent is a type of agreement)
'was sentimental to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'ASSENT' can be found hidden inside 'was sentimental'.
(Other definitions for assent that I've seen before include "Permission" , "Acquiescence" , "Consent or approval" , "Accede, acquiesce" , "Agree and concur" .)