Being both discontented, agonising, end of (4)
I believe the answer is:
aged
'being both' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'discontented agonising end of' is the wordplay.
'discontented' suggests removing the centre (the word\'s contents are removed).
'end of' becomes 'ed' (I am not sure about this - if you are sure you should believe this answer much more).
'agonising' with its middle removed is 'ag'.
'ag'+'ed'='AGED'
Can you help me to learn more?
(Other definitions for aged that I've seen before include "not like a callow youth" , "Ripened, matured" , "Getting on" , "Geriatric" , "Got older" .)