Spoil jewellery, adding mottled effect (8)
I believe the answer is:
marbling
'adding mottled effect' is the definition.
(marbling creates a mottled ink design)
'spoil jewellery' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'jewellery' becomes 'bling' (I've seen this before).
'mar'+'bling'='MARBLING'
(Other definitions for marbling that I've seen before include "Mottled effect" , "Distribution of fat" , "Streaky lines" .)