Diana's engagement is a let-down (14)
I believe the answer is:
disappointment
'a let-down' is the definition.
(I've seen this before)
'diana's engagement' is the wordplay.
'diana's' becomes 'Di's' (Di is short for Diana).
'engagement' becomes 'appointment' (synonyms).
'dis'+'appointment'='DISAPPOINTMENT'
'is' is the link.
(Other definitions for disappointment that I've seen before include "Setback" , "Frustration" , "Frustrating thing" , "Let-down" , "unhappiness" .)