Number with 100 zeroes (6)
I believe the answer is:
googol
'number with 100 zeroes' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for googol that I've seen before include "Ten to the power of 100" , "unimaginable figure" , "There's a huge amount!" , "Big number" , "10100" .)