Big problem with large slick (4)
I believe the answer is:
glib
'slick' is the definition.
(I know that slick can be written as glib)
'big problem with large' is the wordplay.
'problem with' is an anagram indicator.
'large' becomes 'L' (eg in clothes sizes).
'big' anagrammed with 'l' gives 'GLIB'.
(Other definitions for glib that I've seen before include "Of speech, facile but insincere" , "smooth" , "Only superficially plausible" , "Insincere, shallow" , "Fluent and plausible" .)