In one's vintage days? (4)

I believe the answer is:

aged

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

'in' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

'one's vintage days?' is the wordplay.
'one's vintage' becomes 'age'.
'days?' becomes 'd' (abbreviation for days).
'age'+'d'='AGED'

Can you help me to learn more?

(Other definitions for aged that I've seen before include "Got older" , "Became older" , "Time-worn" , "not a modern copy" , "Elderly" .)

I've seen this clue in the Evening Standard.

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.