Declare a sure winner, say (6)
I believe the answer is:
assert
This might be a double definition.
'declare' is the first definition.
(asserting is a kind of declaring)
'a sure winner say' is the second definition.
'assert' can be an answer for 'say' (I've seen this in another clue). I'm not sure about the 'a sure winner' bit.
Can you help me to learn more?
(Other definitions for assert that I've seen before include "Declare, claim" , "State authoritatively" , "State firmly and with conviction" , "State firmly and positively" , "Confidently say" .)