Most certain way to return us the balance (6)
I believe the answer is:
surest
'most certain' is the definition.
('sure' can be a synonym of 'certain')
'return us the balance' is the wordplay.
'return' says the letters should be written backwards.
'the balance' becomes 'rest' (I've seen this before).
'us' backwards is 'su'.
'su'+'rest'='SUREST'
'way to' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for surest that I've seen before include "Most convinced" , "Most certain and confident" , "Most certain or inevitable" , "Most secure" , "With supreme confidence" .)