Fluency, blessing in disguise (8)
I believe the answer is:
glibness
'fluency' is the definition.
(thesaurus)
'blessing in disguise' is the wordplay.
'in disguise' indicates an anagram.
'blessing' with letters rearranged 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" .)