Agree Jacquie's certainly guarding this (9)
I believe the answer is:
acquiesce
'agree' is the definition.
(acquiescing is a kind of agreeing)
'jacquie's certainly guarding this' is the wordplay.
'guarding this' indicates the answer is hidden within the clue.
'ACQUIESCE' is hidden within 'jacquies certainly'.
(Other definitions for acquiesce that I've seen before include "Agree without protest, accept as arrangement" , "Accede, assent" , "Agree without protest, assent" , "consent" , "Give assent" .)