One thus set on fire, a refinery gutted (8)
I believe the answer is:
Here is my best explanation:
'one' is the definition.
'thus set on fire a refinery gutted' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'set on fire' becomes 'lit' (I've seen this before).
'gutted' says to hollow out the word (remove centre letters) (to gut something is to remove its insides).
'refinery' with its centre taken out is 'ry'.
(Other definitions for solitary that I've seen before include "Existing alone", "Confinement", "punishment", "unique", "Lone; isolated", "Isolated; hermit", "On one's own", "Done alone", "Without company" and "One".)