A Sierra left running: fuel is provided (2,4,2)
I believe the answer is:
as long as
'is provided' is the definition.
(both can mean if certain conditions are met)
'a sierra left running fuel' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
'sierra' could be 's' (phonetic alphabet: alpha, bravo, charlie etc.) and 's' is present in the answer.
'left' could be 'l' (common abbreviation) and 'l' is present in the answer.
'running' could be 'on' (eg when a machine is running it is on) and 'on' is found in the answer.
'fuel' could be 'gas' (gas is a kind of fuel) and 'gas' is found within the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for as long as that I've seen before include "Provided" , "for the time" , "On condition that - (" ___ your arm")" .)