Declare positively to be a cert? (6)

I believe the answer is:

assert

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

'declare positively' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for assert that I've seen before include "Aver, declare" , "State firmly, aver" , "State confidently" , "State firmly and with conviction" , "Declare, state vigorously" .)

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.