Poster I designed brings sharp response (7)
I believe the answer is:
riposte
'sharp response' is the definition.
'poster i designed' is the wordplay.
'designed' indicates anagramming the letters (designed with the letters).
'poster'+'i'='posteri'
'posteri' with letters rearranged gives 'RIPOSTE'.
'brings' acts as a link.
(Other definitions for riposte that I've seen before include "Repartee; quick thrust (fencing)" , "Rejoin" , "Quick sharp reply" , "Sally" , "Fencing counterattack" .)