One has 50 left, that's the lot (3)
I believe the answer is:
all
'the lot' is the definition.
(I've seen this in another clue)
'one has 50 left' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'has' is a charade indicator (letters next to each other).
'50' becomes 'l' (Roman numeral).
'left' becomes 'l' (common abbreviation).
'a'+'l'+'l'='ALL'
'that's' acts as a link.
(Other definitions for all that I've seen before include "Garden passage between trees or bushes" , "Everything or everyone" , "The works" , "Quite" , "Comprising every individual one" .)