Emphasise one is not quite out of teens (10)
I believe the answer is:
underscore
'emphasise' is the definition.
(I know that emphasise is a kind of emphasise)
'out of teens' is the wordplay.
'out' becomes 'under' (I've seen this in another clue).
'of teens' becomes 'score' (I can't justify this - if you can you should give a lot more credence to this answer).
'under'+'score'='UNDERSCORE'
'one is not quite' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for underscore that I've seen before include "It's beneath character to highlight" , "Stress" , "Emphasise" , "19 ?" .)