All about acquiring one key colour (5)
I believe the answer is:
lilac
'colour' is the definition.
(lilac is a shade of purple)
'all about acquiring one key' is the wordplay.
'about' is a reversal indicator.
'acquiring' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'key' becomes 'c' (musical key).
'all' backwards is 'lla'.
'lla' enclosing 'i' is 'lila'.
'lila'+'c'='LILAC'
(Other definitions for lilac that I've seen before include "I call (anag)" , "Shrub with fragment purple or white flowers" , "Flower celebrated by Schubert" , "Tree with purple or white flowers" , "Fragrant,purple flower" .)