A poster about grass (7)
I believe the answer is:
esparto
'grass' is the definition.
(type of grass plant)
'a poster about' is the wordplay.
'about' is an anagram indicator.
'a'+'poster'='aposter'
'aposter' anagrammed gives 'ESPARTO'.
(Other definitions for esparto that I've seen before include "Tough, coarse grass" , "Coarse type of grass" , "Source of paper" , "Rope-making process" , "A poster (anag.)" .)