Guarantee concert is ending in Satie (7)
I believe the answer is:
promise
'guarantee' is the definition.
(I know that guarantee is a more specific form of the action promise)
'concert is ending in satie' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'ending in' suggests the final letters.
The final letter of 'satie' is 'e'.
'prom'+'is'+'e'='PROMISE'
(Other definitions for promise that I've seen before include "plight?" , "Vow" , "Pledge - my word!" , "Give one's word" , "Make a commitment" .)