Crossword Genius

Painful reminder of the need for filling time? (9)

Ross

I believe the answer is:

toothache

I'm a little stuck... Click here to teach me more about this clue!

'filling time?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'painful reminder of the need' is the wordplay.
I cannot really see how this works, but
'painful' could be 'ache' (I've seen this before) and 'ache' is found in the answer.
'of' could be 'o' and 'o' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This explanation may well be incorrect...

'for' acts as a link.

Can you help me to learn more?

(Other definitions for toothache that I've seen before include "Pain in the mouth" , "Trouble with choppers" , "Oral discomfort" , "Cause of sore mouth" , "Dental pain" .)

I've seen this clue in The Globe and Mail.
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