Design of poster etc is hush-hush (3,6)
I believe the answer is:
top secret
'hush-hush' is the definition.
(thesaurus)
'design of poster etc' is the wordplay.
'design of' indicates anagramming the letters (I've seen this in other clues).
'poster'+'etc'='posteretc'
'posteretc' with letters rearranged gives 'TOP-SECRET'.
'is' is the link.
(Other definitions for top secret that I've seen before include "Of the highest security classification" , "Erect post (anag)" , "Highest classified (information)" , "label on some classified documents" , "visible to hardly anyone?" .)