My specialist market has no hard decorative moulding (7)
I believe the answer is:
cornice
'decorative moulding' is the definition.
(architectural feature)
'my specialist market has no hard' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'specialist market' becomes 'niche'.
'has no' is a deletion indicator.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'niche' with 'h' removed is 'nice'.
'cor'+'nice'='CORNICE'
(Other definitions for cornice that I've seen before include "Ornamental wall moulding" , "Moulding between ceiling and top of a wall" , "decorative part of room" , "Overhanging ledge of frozen snow" , "Moulding along top of building" .)