Leader of Conservatives in bumbling oaf is humiliating disaster (6)
I believe the answer is:
fiasco
'humiliating disaster' is the definition.
(I've seen this before)
'leader of conservatives in bumbling oaf is' is the wordplay.
'leader of' indicates taking the first letters.
'in' indicates putting letters inside.
'bumbling' indicates anagramming the letters.
The first letter of 'conservatives' is 'c'.
'oaf'+'is'='oafis'
'oafis' anagrammed gives 'fiaso'.
'c' inserted within 'fiaso' is 'FIASCO'.
(Other definitions for fiasco that I've seen before include "Embarrassing failure" , "Ignominious failure" , "washout" , "Debacle, a right mess" , "An absolute mess, what a farce" .)