Nothing once purchased loses billions (5)
I believe the answer is:
ought
'nothing' is the definition.
(I've seen this before)
'purchased loses billions' is the wordplay.
'purchased' becomes 'bought' ('buy' can be a synonym of 'purchase').
'loses' is a deletion indicator.
'billions' becomes 'b' (abbreviation).
'bought' with 'b' taken away is 'OUGHT'.
'once' acts as a link.
(Other definitions for ought that I've seen before include "Be bound" , "Have/has a duty (to do)" , "Tough (anag)" , "Anything at all (archaic)" , "Strange thing" .)