Poster about Clio perhaps was entertaining (6)
I believe the answer is:
amused
'was entertaining' is the definition.
(to amuse is to entertain)
'poster about clio perhaps' is the wordplay.
'poster' becomes 'ad' (short for advert).
'about' indicates putting letters inside.
'clio perhaps' becomes 'muse' (Clio is the muse of history in Greek mythology).
'ad' going around 'muse' is 'AMUSED'.
(Other definitions for amused that I've seen before include "Tickled pink" , "Made (someone) laugh" , "Medusa (anag)" , "Beguiled" , "Queen Victoria wasn't!" .)