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' (common abbreviation for 'with reference to').
'us' backwards is 'su'.
'su'+'re'='SURE'
'for' is the link.
(Other definitions for sure that I've seen before include "Certain to be a ruse" , "Positive" , "Certain, confident" , "Agreed" , "Convinced" .)