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 subsidiary indication.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'set on fire' becomes 'lit' (I've seen this before).
'gutted' suggests removing the centre (to gut something is to remove its insides).
'refinery' with its centre removed is 'ry'.
(Other definitions for solitary that I've seen before include "'Sole, lone (8)'", "Being the only one", "Without company", "Lone", "Isolated; hermit", "Done alone", "On one's own", "Alone, single", "Single" and "Hermit living alone".)