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 a link between them that I don'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" .)