Promise as a certainty (6)

I believe the answer is:

assure

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'promise' is the definition.
('assure' can be a synonym of 'promise')

'as a certainty' is the wordplay.
'a certainty' becomes 'sure' (I can't explain this - if you can you should believe this answer much more).
'as'+'sure'='ASSURE'

Can you help me to learn more?

(Other definitions for assure that I've seen before include "Convince, promise" , "Convince, satisfy" , "Make (something) certain to happen" , "Cover by life insurance" , "Make sure, guarantee" .)

I've seen this clue in the Evening Standard.

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