Adders regularly seen by ancient city ruin (7)
I believe the answer is:
destroy
'ruin' is the definition.
('destroy' can be a synonym of 'ruin')
'adders regularly seen by ancient city' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'seen by' means one lot of letters go next to another.
'ancient city' becomes 'troy' (I've seen this before).
The alternate letters of 'adders' are 'des'.
'des'+'troy'='DESTROY'
(Other definitions for destroy that I've seen before include "Annihilate" , "Wreck" , "Ruin; pull down" , "Soundly defeat" , "Obliterate" .)