Pat of butter in large glass, just starting to turn (4)
I believe the answer is:
glib
'pat' is the definition.
(synonyms)
'butter in large glass just starting to turn' is the wordplay.
'just starting' says to take the initial letters.
'to turn' is a reversal indicator.
The first letters of 'butter in large glass' is 'bilg'.
'bilg' written backwards gives 'GLIB'.
'of' is the link.
(Other definitions for glib that I've seen before include "Superficially facile with words" , "Plausibly fluent" , "Smooth-tongued" , "Of speech, facile but insincere" , "Insincere, shallow" .)