He changes colour (4)
I believe the answer is:
dyer
'he changes' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'colour' is the wordplay.
I cannot quite see how this works, but
'colour' could be 'dye' (dyeing is a kind of colouring) and 'dye' is present in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for dyer that I've seen before include "Cloth-stainer" , "One colouring cloth" , "colourist" , "Cloth stainer" , "He changes colour, as it were" .)