Greek character amongst posters becoming pests (6)
I believe the answer is:
aphids
'pests' is the definition.
(bugs considered pests)
'greek character amongst posters' is the wordplay.
'greek character' becomes 'phi' (letter from the Greek alphabet).
'amongst' means one lot of letters goes inside another (I've seen this in other clues).
'posters' becomes 'ads' (poster is a type of advertisement).
'phi' placed within 'ads' is 'APHIDS'.
'becoming' is the link.
(Other definitions for aphids that I've seen before include "Plant-lice like greenflies" , "Small insects like greenfly or blackfly" , "Plant pests" , "Small insects like greenfly feeding on plants" , "Sucking insects" .)