Not quite a fifty majority (6)
I believe the answer is:
almost
'not quite' is the definition.
(I've seen this before)
'a fifty majority' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'majority' becomes 'most' (most of a group).
'a'+'l'+'most'='ALMOST'
(Other definitions for almost that I've seen before include "Not entirely" , "Well-nigh" , "As good as" , "Virtually" , "Not totally" .)