Alter both reforms in engagement (9)
I believe the answer is:
betrothal
'engagement' is the definition.
(I know that engagement can be written as betrothal)
'alter both reforms' is the wordplay.
'reforms' indicates anagramming the letters.
'alter'+'both'='alterboth'
'alterboth' anagrammed gives 'BETROTHAL'.
'in' acts as a link.
(Other definitions for betrothal that I've seen before include "Plight" , "Pledge" , "Promise of marriage" , "Commitment" , "Engagement to marry" .)