Ruins, lays waste at any rate (9)
I believe the answer is:
leastways
'at any rate' is the definition.
('leastways' can be a synonym of 'at any rate')
'ruins lays waste' is the wordplay.
'ruins' indicates an anagram.
'lays'+'waste'='layswaste'
'layswaste' with letters rearranged gives 'LEASTWAYS'.
(Other definitions for leastways that I've seen before include "If nothing else" , "at any rate" , "As a minimum" , "anyhow" , "in any event" .)