Coming back over, US sailor keeps promise (6)
I believe the answer is:
assure
'promise' is the definition.
('assure' can be a synonym of 'promise')
'coming back over us sailor keeps' is the wordplay.
'coming back' is a reversal indicator.
'keeps' says the answer is hidden in the clue.
'erussa' is hidden within 'over us sailor'.
'erussa' in reverse letter order is 'ASSURE'.
(Other definitions for assure that I've seen before include "Convince, guarantee" , "Convince, satisfy" , "Cover by life insurance" , "Cover with a policy" , "Convince, promise" .)