PM once was disheartened and upset leaving country (4)
I believe the answer is:
eden
'pm once' is the definition.
(Anthony Eden, UK prime minister 1955-1957)
'was disheartened and upset leaving country' is the wordplay.
'was' becomes 'existed' (past tense of to be - to exist).
'disheartened' says to hollow out the word (remove centre letters) (the middle or heart is removed).
'and' says to put letters next to each other.
'upset leaving country' becomes 'en' (I can't justify this - if you can you should give a lot more credence to this answer).
'existed' with its middle removed is 'ed'.
'ed'+'en'='EDEN'
Can you help me to learn more?
(Other definitions for eden that I've seen before include "In need of a great garden" , "Unspoiled paradise" , "Anthony ---, PM 1955" , "Paradise - ex-PM" , "Place of complete bliss" .)