Article supporting politician in gaol leads UN to act prematurely (4,3,3)
I believe the answer is:
jump the gun
'act prematurely' is the definition.
(I've seen this before)
'article supporting politician in gaol leads un' is the wordplay.
'article' becomes 'the' (the definite article in English).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'politician in' becomes 'jump' (I can't explain this - if you can you should believe this answer much more).
'leads' indicates taking the first letters.
The initial letter of 'gaol' is 'g'.
'the' after 'jump' is 'jumpthe'.
'jumpthe'+'g'+'un'='JUMP THE GUN'
'to' acts as a link.
(Other definitions for jump the gun that I've seen before include "Come too early out of the starting blocks?" , "Start too early" , "Act prematurely" , "Be too hasty" , "Leave too early" .)