Ring up about one in a light purple colour (5)
I believe the answer is:
lilac
'a light purple colour' is the definition.
(I've seen this before)
'ring up about one in' is the wordplay.
'ring up' becomes 'call' (I've seen this before).
'about' says the letters should be written backwards.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'call' reversed gives 'llac'.
'llac' enclosing 'i' is 'LILAC'.
(Other definitions for lilac that I've seen before include "Shrub or small tree with fragrant blossom" , "Light purple - flowering tree" , "Shrub with fragment purple or white flowers" , "Pale purple - flowering tree" , "Fragrant shrub; colour" .)