One hundred to an indefinite number (3)
I believe the answer is:
ton
'one hundred' is the definition.
(I've seen this before)
'to an indefinite number' is the wordplay.
'an indefinite number' becomes 'n' (I've seen this in other clues).
'to'+'n'='TON'
(Other definitions for ton that I've seen before include "Twenty cwt" , "High-speed" , "Century" , "20cwt" , "Weight - fashion (French)" .)