You'll notice, now and then, that it teeters (6)
I believe the answer is:
seesaw
'you'll' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?
'notice now and then that it teeters' is the wordplay.
I cannot quite see how this works, but
'notice' could be 'see' (notice can mean to spot or see) and 'see' is located in the answer.
'it' could be 'sa' (old-fashioned abbreviation for sex appeal) and 'sa' is present in the answer.
A single letter 'w' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for seesaw that I've seen before include "Children's up-and-down play structure" , "Fluctuating playground ride" , "Pivoted amusement" , "Go one way and then the other" , "Plank children play on" .)