Ornament used for tile design (7)
I believe the answer is:
trefoil
'ornament' is the definition.
(trefoil is a kind of ornament)
'for tile design' is the wordplay.
'design' indicates anagramming the letters.
'for'+'tile'='fortile'
'fortile' is an anagram of 'TREFOIL'.
'used' is the link.
(Other definitions for trefoil that I've seen before include "Plant with three leaves, like shamrock or clover" , "A flower; three-lobe ornament" , "Plant with leaves in three parts, eg clover" , "Small yellow-flowered plant" , "Plant with clover-like leaves" .)