One has 50 left, that's the lot (3)
I believe the answer is:
all
'the lot' is the definition.
(I've seen this before)
'one has 50 left' is the wordplay.
'one' becomes 'a' ().
'has' says to put 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" .)