Adders regularly seen by ancient city ruin (7)
I believe the answer is:
Here is my best explanation:
'ruin' is the definition.
('destroy' can be a synonym of 'ruin')
'adders regularly seen by ancient city' is the subsidiary indication.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'seen by' says to put letters next to each other.
'ancient city' becomes 'troy' (I've seen this before).
The alternating letters of 'adders' are 'des'.
(Other definitions for destroy that I've seen before include "Obliterate", "Soundly defeat", "Ruin, put an end to", "Flatten", "Kill; ruin", "Completely ruin", "put to death", "Demolish; pull down", "End" and "Put an end to; ruin".)