Origin of decay around hole? (4)
I believe the answer is:
root
'origin' is the definition.
('root' can be a synonym of 'origin')
'decay around hole?' is the wordplay.
'decay' becomes 'rot' (rot is a kind of decay).
'around' is an insertion indicator.
'hole?' becomes 'o'.
'rot' enclosing 'o' is 'ROOT'.
'of' is the link.
(Other definitions for root that I've seen before include "Basic source or origin" , "Turn up with the snout." , "support" , "Basic cause or origin" , "burrow" .)