Cost to fill tank (6)
I believe the answer is:
charge
This might be a double definition.
'cost' is the first definition.
(charge is a kind of cost)
'fill tank' is the second definition.
'charge' can be an answer for 'fill' (charging is a kind of filling). I am unsure of the 'tank' bit.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for charge that I've seen before include "Amount to pay - attack" , "Control" , "Move quickly and violently" , "Ask for" , "damage" .)