The source yields nothing and I ring off (6)
I believe the answer is:
origin
'the source' is the definition.
('origin' can be a synonym of 'source')
'nothing and i ring off' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'and' is a charade indicator (letters next to each other).
'off' indicates anagramming the letters.
'i'+'ring'='iring'
'iring' with letters rearranged gives 'rigin'.
'o'+'rigin'='ORIGIN'
'yields' acts as a link.
(Other definitions for origin that I've seen before include "Place or point of beginning (6)" , "Square one" , "First thing" , "Starting point or cause" , "Source, derivation" .)