Spoil only half the velvet - it's a wonder! (6)
I believe the answer is:
marvel
'it's a wonder' is the definition.
('marvel' can be a synonym of 'wonder')
'spoil only half the velvet' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'only half' indicates taking half.
'velvet' with half the letters taken is 'vel'.
'mar'+'vel'='MARVEL'
(Other definitions for marvel that I've seen before include "Be filled with surprise or wonder" , "Express wonder" , "Wonderful or astonishing person" , "Cause of wonder" , "A wonderful or remarkable thing" .)