Drive-in so funny causing amusement (9)
I believe the answer is:
'amusement' is the definition.
(I've seen this before)
'drive-in so funny' is the wordplay.
'funny' indicates an anagram.
'driveinso' with letters rearranged gives 'DIVERSION'.
'causing' acts as a link.
(Other definitions for diversion that I've seen before include "entertainment" , "Detour" , "Alternative route" , "Amusement -- temporary redirection" .)