Agreed to become old if disheartened (4)
I believe the answer is:
aged
'disheartened' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'agreed to become old' is the wordplay.
'agreed' becomes 'd' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'become old' becomes 'age' (to age is to get older).
'd' put after 'age' is 'AGED'.
'if' is the link.
Can you help me to learn more?
(Other definitions for aged that I've seen before include "Long in the tooth" , "Geriatric" , "Time-worn" , "Grew old" , "Very old, person or wine" .)