Walls of emerald upset my colour scheme (5)
I believe the answer is:
decor
'colour scheme' is the definition.
(I've seen this before)
'walls of emerald upset my' is the wordplay.
'walls of' says to hollow out the word (remove centre letters) (only the edge letters remain).
'upset' says the letters should be written backwards.
'my' becomes 'cor' (both are expressions of surprise).
'emerald' with its middle removed is 'ed'.
'ed' in reverse letter order is 'de'.
'de'+'cor'='DECOR'
(Other definitions for decor that I've seen before include "stage background" , "Embellishment of a room" , "Colour-scheme etc of a room" , "Interior layout and furnishings" , "Colour scheme, furnishing etc. of a room" .)