A plus, without doubt (8)

I believe the answer is:

positive

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

'a plus' is the definition.
(I know that plus can be written as positive)

I don't understand the rest of the clue.

(Other definitions for positive that I've seen before include "Confident, very sure" , "Of a number, greater than zero - that's certain" , "Definite, confident" , "Optimistic; certain" , "Constructive, optimistic" .)

I've seen this clue in the Evening Standard.

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