Ruin in Dorset Yankee rebuilt (7)
I believe the answer is:
destroy
'ruin' is the definition.
(I know that ruin can be written as destroy)
'dorset yankee rebuilt' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'rebuilt' indicates anagramming the letters.
'dorset'+'y'='dorsety'
'dorsety' is an anagram of 'DESTROY'.
'in' acts as a link.
(Other definitions for destroy that I've seen before include "Flatten" , "Demolish; pull down" , "Completely ruin" , "Ruin; pull down" , "Ruin, put an end to" .)