Colourful display — one covered by a brown form (7)
I believe the answer is:
rainbow
'colourful display' is the definition.
(spectrum of colours)
'one covered by a brown form' is the wordplay.
'one' becomes 'i' (Roman numeral).
'covered by' means one lot of letters goes inside another.
'form' is an anagram indicator (a different form of the same letters).
'a'+'brown'='abrown'
'abrown' is an anagram of 'ranbow'.
'i' placed into 'ranbow' is 'RAINBOW'.
(Other definitions for rainbow that I've seen before include "colours in the sky" , "an impossible dream being chased?" , "All the colours of the spectrum" , "'I can sing a - - ' (song)" , "Phenomenon produced by light passing through water droplets" .)