Promise that makes both alter (9)
I believe the answer is:
betrothal
'promise' is the definition.
(betrothal is a kind of promise)
'makes both alter' is the wordplay.
'makes' is an anagram indicator.
'both'+'alter'='bothalter'
'bothalter' anagrammed gives 'BETROTHAL'.
'that' is the link.
(Other definitions for betrothal that I've seen before include "Promise between two people" , "Pledge" , "Plight" , "Promise of marriage" , "Engagement to marry" .)