Claim a sure thing, so we hear (6)
I believe the answer is:
assert
'claim a sure thing' is the definition.
'assert' can be an answer for 'claim' (I've seen this before). I am not certain of the 'a sure thing' bit.
'so we hear' is the wordplay.
'so' becomes 'as' (e.g. phrases like 'not as tall' and 'not so tall').
'we hear' becomes 'sert' (I can't explain this - if you can you should believe this answer much more).
'as'+'sert'='ASSERT'
(Other definitions for assert that I've seen before include "Declare, profees" , "State confidently" , "State categorically" , "Establish control or state as true" , "Maintain formally" .)