Poster designed quickly (6)
I believe the answer is:
presto
'quickly' is the definition.
(I've seen this before)
'poster designed' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'poster' with letters rearranged gives 'PRESTO'.
(Other definitions for presto that I've seen before include "(In music) fast" , "scored quickly" , "To be played quickly" , "Hey, quickly for a magician" , "fast to the maximum" .)