Shock retrospective rescued gallery (9)
I believe the answer is:
devastate
'shock' is the definition.
(thesaurus)
'retrospective rescued gallery' is the wordplay.
'retrospective' says the letters should be written in reverse.
'rescued' becomes 'saved' (saving is a kind of rescuing).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'saved' back-to-front is 'devas'.
'devas'+'tate'='DEVASTATE'
(Other definitions for devastate that I've seen before include "Destroy" , "Ruin" , "Lay waste or totally crush" , "Spoil" , "floor" .)