Purple heads in magenta and emerald, absorbing ultraviolet (5)
I believe the answer is:
mauve
'purple' is the definition.
(mauve is a kind of purple)
'heads in magenta and emerald absorbing ultraviolet' is the wordplay.
'heads in' suggests taking the first letters (I've seen 'head in' mean this).
'absorbing' means one lot of letters goes inside another.
'ultraviolet' becomes 'uv' (abbreviation).
The initial letters of 'magenta and emerald' is 'mae'.
'mae' placed around 'uv' is 'MAUVE'.
(Other definitions for mauve that I've seen before include "sort of light seen in West" , "Light purple" , "Shade of pale purple" , "Bluish purple" , "Purplish colour" .)