Beer has Meg changing colour (7)
I believe the answer is:
pigment
'colour' is the definition.
(I know that pigment is a type of colour)
'beer has meg changing' is the wordplay.
'beer' becomes 'pint' (beer is typically served by the pint).
'has' indicates putting letters inside.
'changing' indicates anagramming the letters.
'meg' with letters rearranged gives 'gme'.
'pint' enclosing 'gme' is 'PIGMENT'.
(Other definitions for pigment that I've seen before include "Paint" , "Substance giving colour" , "Colour or dye" , "Colourful substance" , "Colouring material" .)