Chopping up a red elm reveals colour (7)
I believe the answer is:
emerald
'colour' is the definition.
(emerald is a kind of colour)
'chopping up a red elm' is the wordplay.
'chopping up' is an anagram indicator (I've seen 'chopped up' mean this).
'a'+'red'+'elm'='aredelm'
'aredelm' with letters rearranged gives 'EMERALD'.
'reveals' acts as a link.
(Other definitions for emerald that I've seen before include "Highly-valued variety of beryl" , "Gem; type of moth" , "Green variety of beryl" , "Gem associated with Ireland" , "Green gemstone" .)