Expose United Nations fellow corrupt at heart (7)
I believe the answer is:
uncover
'expose' is the definition.
('uncover' can be a synonym of 'expose')
'united nations fellow corrupt at heart' is the wordplay.
'united nations' becomes 'un' ('UN' can be a synonym of 'united nations').
'fellow' becomes 'cove' (British slang term**).
'at heart' means to look at the middle letters.
The centre of 'corrupt' is 'r'.
'un'+'cove'+'r'='UNCOVER'
(Other definitions for uncover that I've seen before include "bare" , "show?" , "make a revelation" , "Reveal - take top off" , "Expose, reveal" .)