Colour is arguably absent, oddly (4)
I believe the answer is:
ruby
'colour' is the definition.
(I know that ruby is a colour)
'arguably absent oddly' is the wordplay.
'absent oddly' indicates alternate letters (letters in odd-numbered positions removed).
The alternating letters of 'arguably' are 'RUBY'.
'is' acts as a link.
(Other definitions for ruby that I've seen before include "Red precious stone" , "A red jewel" , "Deep 6 across" , "Completely" , "Deep red variety of corundum" .)