Ruin Dave's new gallery (9)
I believe the answer is:
devastate
'ruin' is the definition.
(I know that devastate is a is a more specific form of the action of ruin)
'dave's new gallery' is the wordplay.
'new' indicates anagramming the letters.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'daves' is an anagram of 'devas'.
'devas'+'tate'='DEVASTATE'
(Other definitions for devastate that I've seen before include "Destroy" , "Lay waste or totally crush" , "floor" , "Spoil" , "Lay waste or overwhelm with grief" .)