Do away with hollow effect round moulding (7)
I believe the answer is:
annulet
'round moulding' is the definition.
(type of round architectural feature)
'do away with hollow effect' is the wordplay.
'do away with' becomes 'annul' (to annul is to cancel or repeal).
'hollow' means to remove the middle letters.
'effect' with its middle removed is 'et'.
'annul'+'et'='ANNULET'
(Other definitions for annulet that I've seen before include "Heraldic small ring" , "Band around a column; heraldic ring" , "Small ring (heraldry)" , "small charge" , "Small band around a column" .)