Mere lad wearing jewel (7)
I believe the answer is:
emerald
'jewel' is the definition.
(I know that emerald is a type of jewel)
'mere lad wearing' is the wordplay.
'wearing' is an anagram indicator (wear can mean to become damaged).
'mere'+'lad'='merelad'
'merelad' is an anagram of 'EMERALD'.
(Other definitions for emerald that I've seen before include "a type employed by the press" , "Highly-valued variety of beryl" , "Ireland is often called The .......Isle" , "Green gemstone" , "Small size of type, in printing" .)