Now that I think about it, no! (2,10)

'now' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see a link between them that I don't see?

'i think about it no' is the wordplay.
I cannot really understand how this works, but
'i' is present in the answer.
'think' could be 'reflect' (reflecting is a kind of thinking) and 'reflect' is found in the answer.
'about' could be 'on' (as in a book 'on' a certain topic) and 'on' is found within the answer.
'no' could be 'o' (O resembles 0) and 'o' is found in the answer.
A single letter 'n' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

'that' acts as a link.

Can you help me to learn more?