... natter with eminent lunatic provides amusement (13)
I believe the answer is:
entertainment
'amusement' is the definition.
(synonyms)
'natter with eminent lunatic' is the wordplay.
'with' means one lot of letters go next to another.
'lunatic' indicates an anagram.
'natter' after 'eminent' is 'eminentnatter'.
'eminentnatter' with letters rearranged gives 'ENTERTAINMENT'.
'provides' acts as a link.
(Other definitions for entertainment that I've seen before include "show business" , "sport" , "Source of amusement" , "Pleasure-giving" , "Public show; hospitality" .)