Confirm fuel returned after five and before end of day (6)
I believe the answer is:
verify
'confirm' is the definition.
(I know that verify is a kind of confirm)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for verify that I've seen before include "Prove to be true" , "Check to be true" , "Establish truth or accuracy of" , "Demonstrate" , "Authenticate" .)