One who has fallen in love is revealed by a change in face (6)
I believe the answer is:
fiance
'one who has fallen in love is revealed' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a change in face' is the wordplay.
'a change' indicates an anagram.
'in'+'face'='inface'
'inface' with letters rearranged gives 'FIANCE'.
'by' is the link.
Can you help me to learn more?
(Other definitions for fiance that I've seen before include "One who's engaged" , "One engaged to marry" , "Prospective groom" , "One promised in marriage" , "Groom-to-be" .)