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' ('re' means regarding or about).
'us' backwards is '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" .)