Keir, in short, the ablest in Labour? Nonsense! (12)
I believe the answer is:
blatherskite
'nonsense' is the definition.
(blatherskite is a kind of nonsense)
'keir in short the ablest in labour?' is the wordplay.
'in short' means to remove the last letter.
'in labour?' indicates an anagram (I've seen this in other clues).
'the' with its last letter removed is 'th'.
'keir'+'th'+'ablest'='keirthablest'
'keirthablest' with letters rearranged gives 'BLATHERSKITE'.
(Other definitions for blatherskite that I've seen before include "Chatterer" , "(Person talking) nonsense -- a Lettish berk (anag)" , "pants" , "fool" .)