Top entertaining line by vulgar speaker is guff (8)
I believe the answer is:
claptrap
'is guff' is the definition.
(thesaurus)
'top entertaining line by vulgar speaker' is the wordplay.
'top' becomes 'cap' (both can mean to beat or outdo).
'entertaining' means one lot of letters goes inside another (inviting in, as for dinner).
'line' becomes 'l' (used when specifying particular lines from a poem).
'by' says to put letters next to each other.
'vulgar speaker' becomes 'trap' (I am not sure about this - if you are sure you should believe this answer much more).
'cap' enclosing 'l' is 'clap'.
'clap'+'trap'='CLAPTRAP'
Can you help me to learn more?
(Other definitions for claptrap that I've seen before include "Foolish or pretentious talk" , "Pretentious talk or nonsense" , "High-sounding nonsense" , "moonshine" , "poppycock" .)