O2 deleting digit in writer's long number (6)
I believe the answer is:
googol
'long number' is the definition.
(number with 100 zeroes)
'o2 deleting digit in writer's' is the wordplay.
I cannot really understand how this works, but
'o2' could be 'oo' (Two o's) and 'oo' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for googol that I've seen before include "Ten to the power of 100" , "very large number" , "extraordinary figure" , "10100" , "unimaginable figure" .)