Left the thing switched on (3)
I believe the answer is:
lit
'switched on' is the definition.
The definition and answer can be both related to change as well as being past participle verbs.
Perhaps you can see an association between them that I can't see?
'left the thing' is the wordplay.
'left' becomes 'l' (common abbreviation).
'the thing' becomes 'it'.
'l'+'it'='LIT'
Can you help me to learn more?
(Other definitions for lit that I've seen before include "Set afire" , "Set fire to" , "Illuminated" , "books etc?" , "landed here" .)