Fuel line is operating at first after set-up (3)
I believe the answer is:
oil
'fuel' is the definition.
(thesaurus)
'line is operating at first after set-up' is the wordplay.
'line' becomes 'l'.
'at first' indicates taking the first letters.
'after' is a charade indicator (letters next to each other).
'set up' is a reversal indicator.
The initial letters of 'is operating' is 'io'.
'l'+'io'='lio'
'lio' back-to-front is 'OIL'.
(Other definitions for oil that I've seen before include "Type of fuel" , "painting" , "Valuable viscous liquid" , "Fossil fuel" , "Lubricant or petroleum" .)