Stumbling into ruins - trespass? (9)
I believe the answer is:
intrusion
'trespass?' is the definition.
('intrusion' can be a synonym of 'trespass')
'stumbling into ruins' is the wordplay.
'stumbling' is an anagram indicator.
'into'+'ruins'='intoruins'
'intoruins' is an anagram of 'INTRUSION'.
(Other definitions for intrusion that I've seen before include "Uninvited presence" , "Encroachment" , "Something entering undesirably" .)