Turn us about, for certain (4)
I believe the answer is:
sure
'certain' is the definition.
(synonyms)
'turn us about' is the wordplay.
'turn' says the letters should be written backwards.
'about' becomes 're' (regarding).
'us' written backwards gives 'su'.
'su'+'re'='SURE'
'for' acts as a link.
(Other definitions for sure that I've seen before include "Certain to be a ruse" , "Positive" , "Certain, confident" , "Agreed" , "Convinced" .)