It's what happens after you reach 999 (1,8)

'reach 999' is the definition.
I can't tell whether this definition defines the answer.

'it's what happens after you' is the wordplay.
I cannot quite understand how this works, but
'you' could be 'thou' (historical second person pronoun) and 'thou' is found within the answer.
'it' could be 'sa' (SA can mean sex appeal, as in 'it') and 'sa' is found in the answer.
The remaining letters 'and' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?