Sets up outside. Acceptable, but adds fuel to the fire (6)
I believe the answer is:
stokes
'adds fuel to the fire' is the definition.
I can't tell whether this definition defines the answer.
'sets up outside acceptable' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'outside' indicates putting letters inside.
'acceptable' becomes 'ok' (OK can mean acceptable or fine).
'sets' reversed gives 'stes'.
'stes' enclosing 'ok' is 'STOKES'.
'but' is the link.
Can you help me to learn more?
(Other definitions for stokes that I've seen before include "Feeds (a furnace)" , "Shovels coal into furnace" , "Viscosity units; fuels (furnace)" , "Feeds with fuel" , "Adds coal to (a furnace)" .)