So 'Tripe!' is silly retort (7)
I believe the answer is:
riposte
'retort' is the definition.
(I know that retort can be written as riposte)
'so tripe is silly' is the wordplay.
'is silly' is an anagram indicator.
'so'+'tripe'='sotripe'
'sotripe' with letters rearranged gives 'RIPOSTE'.
(Other definitions for riposte that I've seen before include "Quick retort or return thrust in fencing" , "Quick clever reply" , "Counter-thrust" , "Fencer's counterattack" , "Quick retort or return hit in fencing" .)