Crossword Genius

Not old enough for a state nurse (3)

Ross

I believe the answer is:

ten

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

'not old enough' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'a state nurse' is the wordplay.
'TEN' is hidden within the letters.
However, I am unsure how the hidden word is indicated.

'for' acts as a link.

Can you help me to learn more?

(Other definitions for ten that I've seen before include "How many" , "Five times two" , "Base of decimal system" , "X-WORD" , "Half a score" .)

I've seen this clue in the Evening Standard.
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