It's presumably been growing longer (5)

I believe the answer is:

elder

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

'growing longer' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for elder that I've seen before include "Senior churchperson" , "Older or two" , "comparatively aged" , "White-flowered tree" , "Character of church" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app