Guarantee concert is free at last (7)
I believe the answer is:
promise
'guarantee' is the definition.
(thesaurus)
'concert is free at last' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'at last' says to take the final letters.
The last letter of 'free' is 'e'.
'prom'+'is'+'e'='PROMISE'
(Other definitions for promise that I've seen before include "Pledge - my word!" , "Oath" , "Assurance" , "Hope; commitment" , "Give one's word" .)