Garden site — it's abandoned and decays (13)
I believe the answer is:
disintegrates
'decays' is the definition.
('disintegrate' can be a synonym of 'decay')
'garden site it's abandoned' is the wordplay.
'abandoned' is an anagram indicator.
'garden'+'site'+'its'='gardensiteits'
'gardensiteits' is an anagram of 'DISINTEGRATES'.
'and' is the link.
(Other definitions for disintegrates that I've seen before include "Falls apart" , "Breaks apart" .)