Amount of light ready on odd occasions (3)
I believe the answer is:
ray
This could be a double definition.
'amount of light' is the first definition.
'ready on odd occasions' is the second definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Maybe you can see an association between them that I can't see?
(Other definitions for ray that I've seen before include "Beam or flatfish" , "Beam of light or flat fish" , "Shaft, beam" , "Man, maybe" , "Beam; man's name" .)