Poster associated with old pantomime (3)
I believe the answer is:
ado
'pantomime' is the definition.
(both can mean a ridiculous situation)
'poster associated with old' is the wordplay.
'poster' becomes 'ad' (short for advert).
'associated with' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ad'+'o'='ADO'
(Other definitions for ado that I've seen before include "Hoo-ha" , "carry-on" , "flapping" , "Fuss or bustle" , "Pother" .)