Mere lad might be this green? (7)
I believe the answer is:
emerald
'this green?' is the definition.
(emerald is a kind of green)
'mere lad might be' is the wordplay.
'might be' indicates anagramming the letters.
'mere'+'lad'='merelad'
'merelad' is an anagram of 'EMERALD'.
(Other definitions for emerald that I've seen before include "and 1 Across: Ireland" , "Green variety of beryl" , "Led mare (anag.)" , "a type employed by the press" , "Gem; type of moth" .)