Ex leaders of organisation let down, feeling pathetic (3,5)
I believe the answer is:
old flame
'ex' is the definition.
(I've seen this before)
'leaders of organisation let down feeling pathetic' is the wordplay.
'leaders of' suggests taking the first letters.
'pathetic' becomes 'lame' (I've seen this in another clue).
The first letters of 'organisation let down feeling' is 'oldf'.
'oldf'+'lame'='OLD FLAME'
(Other definitions for old flame that I've seen before include "Former lover" , "Former partner" , "still 1 across [ALIGHT], it seems" , "Ex" , "Someone once loved" .)