Chap finds ancient city ruin (7)
I believe the answer is:
destroy
'ruin' is the definition.
(I know that ruin is a more specific form of desolate)
'chap finds ancient city' is the wordplay.
'chap' becomes 'des' ().
'finds' means one lot of letters go next to another.
'ancient city' becomes 'troy' (Troy was an ancient city).
'des'+'troy'='DESTROY'
(Other definitions for destroy that I've seen before include "Kill; ruin" , "Obliterate" , "put to death" , "Ruin; pull down" , "Demolish; pull down" .)