Wow: lovely piece of plasterwork (7)
I believe the answer is:
cornice
'piece of plasterwork' is the definition.
(piece of plasterwork on a ceiling)
'wow lovely' is the wordplay.
'wow' becomes 'cor' (both are expressions of surprise).
'lovely' becomes 'nice' (I've seen this before).
'cor'+'nice'='CORNICE'
(Other definitions for cornice that I've seen before include "decorative part of room" , "Ledge of snow overhanging a precipice" , "Ornamental wall moulding" , "Moulding between ceiling and top of a wall" , "Moulding along top of building" .)