You’ll be sorry when it comes! (3,3,2,9)

'you'll' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'sorry when it comes' is the wordplay.
I cannot really see how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

'be' is the link.

Can you help me to learn more?