Silver-tongued quality, mixed blessing (8)
I believe the answer is:
glibness
'silver-tongued quality' is the definition.
(the quality of smooth talking)
'mixed blessing' is the wordplay.
'mixed' indicates an anagram.
'blessing' anagrammed gives 'GLIBNESS'.
(Other definitions for glibness that I've seen before include "Blessing (anag) - quality of a slick sales person" , "Blessing (anag) -- slickness of speech" .)