Singer mostly dressing formally? (5)

I believe the answer is:

robin

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

'singer' is the definition.
(robins are songbirds)

I cannot understand how the rest of the clue works.

(Other definitions for robin that I've seen before include "Christmas songbird" , "See 7" , "Batman's assistant" , "Hood, possibly" , ". . . . . Hood was a medieval outlaw" .)

I've seen this clue in The Times.

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