Fast-talking major holding line up (4)
I believe the answer is:
glib
'fast-talking' is the definition.
(I've seen this in another clue)
'major holding line up' is the wordplay.
'major' becomes 'big' (I've seen this in other clues).
'holding' indicates putting letters inside.
'line' becomes 'l' (used when specifying particular lines from a poem).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'big' placed around 'l' is 'bilg'.
'bilg' written backwards gives 'GLIB'.
(Other definitions for glib that I've seen before include "smooth" , "Only superficially plausible" , "Insincerely fluent" , "Ready with insincere or superficial words" , "Smooth-talking" .)