Fed up, former top diplomat collapsed (8)
I believe the answer is:
deflated
'collapsed' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'fed up former top diplomat' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'former' becomes 'late' (late can mean former or previous).
'top' indicates taking the first letters (in a down clue, the first letter is top).
The initial letter of 'diplomat' is 'd'.
'fed' written backwards gives 'def'.
'def'+'late'+'d'='DEFLATED'
Can you help me to learn more?
(Other definitions for deflated that I've seen before include "Let out air or gas from" , "Lost self-esteem - or gas?" , "Put down" , "Depressed like punctured balloon" , "Crestfallen" .)