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