Rush to fill up (6)
I believe the answer is:
charge
This clue is a double definition.
'rush' is the first definition.
(charging is a kind of rushing)
'fill up' is the second definition.
(charging is a kind of filling up)
'to' is the link.
(Other definitions for charge that I've seen before include "Demand price; accuse" , "running cost" , "Rush to exact payment?" , "Be aggressive" , "damage" .)