You'll observe, now and then, that it balances (6)
I believe the answer is:
seesaw
'you'll' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I don't see?
'observe now and then that it balances' is the wordplay.
I cannot quite see how this works, but
'observe' could be 'see' (observe can mean to see something) and 'see' is present in the answer.
'it' could be 'sa' (SA can mean sex appeal, as in 'it') and 'sa' is found in the answer.
A single letter 'w' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for seesaw that I've seen before include "Teeter-totter" , "Move up and down on the plaything" , "that has its ups and downs" , "Child's plaything with plank over barrel" , "'Move up and down alternately'" .)