The "F" on a gas gauge (4)
I believe the answer is:
full
'the f on a gas gauge' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for full that I've seen before include "(Of a figure) plump or rounded" , "Able to eat no more" , "At maximum capacity" , "Containing as much as possible" , "Sated" .)