Nearly all lost a pound – the majority (6)
I believe the answer is:
almost
'nearly' is the definition.
('almost' can be a synonym of 'nearly')
'all lost a pound the majority' is the wordplay.
'lost' indicates named letters should be taken away.
'a pound' becomes 'l'.
'the majority' becomes 'most' (I've seen this before).
'all' with 'l' removed is 'al'.
'al'+'most'='ALMOST'
(Other definitions for almost that I've seen before include "Virtually" , "Slightly short of" , "Not entirely" , "Well-nigh" , "practically" .)