Colour of a red elm that's worked (7)
I believe the answer is:
emerald
'colour' is the definition.
(emerald is a kind of colour)
'a red elm that's worked' is the wordplay.
'that's worked' is an anagram indicator.
'a'+'red'+'elm'='aredelm'
'aredelm' with letters rearranged gives 'EMERALD'.
'of' acts as a link.
(Other definitions for emerald that I've seen before include "Bright green colour" , "Small size of type, in printing" , "Green - jewel" , "Ireland is often called The .......Isle" , "Green gemstone" .)