Stain resulting from decay, reportedly (3)
I believe the answer is:
dye
'stain' is the definition.
(I've seen this before)
'decay reportedly' is the wordplay.
'decay' becomes 'dy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'reportedly' becomes 'e' (I've seen this in other clues).
'dy'+'e'='DYE'
'resulting from' is the link.
Can you help me to learn more?
(Other definitions for dye that I've seen before include "Colouring agent" , "Staining material" , "Soluble substance for staining or colouring" , "Tint" , "Pigment" .)