One reinstated in the team? (4)

I believe the answer is:

back

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

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for back that I've seen before include "Help!" , "Place where secret deals are done" , "Support; footballer" , "part of the body" , "Fashionable again" .)

I've seen this clue in the Evening Standard.

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