It's an odd discolouration (5)
I believe the answer is:
stain
'discolouration' is the definition.
('stain' can be a synonym of 'discolouration')
'it's an odd' is the wordplay.
'odd' is an anagram indicator.
'its'+'an'='itsan'
'itsan' with letters rearranged gives 'STAIN'.
(Other definitions for stain that I've seen before include "Discolouring mark" , "Mark with colour, dirt" , "Satin (anag.)" , "Discolouration" , "Discoloured patch" .)