Not quite a 50 majority (6)
I believe the answer is:
almost
'not quite' is the definition.
(I've seen this before)
'a 50 majority' is the wordplay.
'50' becomes 'l' (roman numeral).
'majority' becomes 'most' (I've seen this before).
'a'+'l'+'most'='ALMOST'
(Other definitions for almost that I've seen before include "Nearly, not quite" , "practically" , "Virtually" , "Well-nigh" , "Not totally" .)