Left the thing burning (3)
I believe the answer is:
lit
'burning' is the definition.
(I've seen this before)
'left the thing' is the wordplay.
'left' becomes 'L' (common abbreviation).
'the thing' becomes 'it'.
'l'+'it'='LIT'
(Other definitions for lit that I've seen before include "Set alight" , "landed here" , "was illuminating" , "Enlightened" , "Set afire" .)