Led mare out. What a gem! (7)
I believe the answer is:
emerald
'what a gem' is the definition.
(type of gem stone)
'led mare out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'led'+'mare'='ledmare'
'ledmare' is an anagram of 'EMERALD'.
(Other definitions for emerald that I've seen before include "Highly-valued variety of beryl" , "and 1 Across: Ireland" , "Green like this isle" , "Led mare (anag.)" , "Small size of type, in printing" .)