Top number is one less than twenty (4)

I believe the answer is:

teen

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

'one less than twenty' is the definition.
I can't judge whether this definition defines the answer.

'top number' is the wordplay.
'top' becomes 'tee' (T-shirt).
'number' becomes 'n' (n can represent any number).
'tee'+'n'='TEEN'

'is' is the link.

Can you help me to learn more?

(Other definitions for teen that I've seen before include "Juvenile" , "Adolescent (informal)" , "Of 13 to 19 year olds" , "Youngster over twelve" , "Youth" .)

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.