Crossword Genius

Highlight when still a teenager? (10)

Ross

I believe the answer is:

underscore

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'highlight' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

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(Other definitions for underscore that I've seen before include "19 ?" , "It's beneath character to highlight" , "Emphasise" , "Stress" .)

I've seen this clue in The Guardian.
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