Facile drivel mostly rejected (4)
I believe the answer is:
glib
'facile' is the definition.
(thesaurus)
'drivel mostly rejected' is the wordplay.
'drivel' becomes 'bilge' (I've seen this before).
'mostly' means to remove the last letter.
'rejected' is a reversal indicator.
'bilge' with its final letter taken away is 'bilg'.
'bilg' in reverse letter order is 'GLIB'.
(Other definitions for glib that I've seen before include "Superficially facile with words" , "slick in speech" , "Only superficially plausible" , "Casually fluent" , "Smooth-talking" .)