What posters need may be a right to wear hat (6,3)
I believe the answer is:
pillar box
'what posters need' is the definition.
(people posting letters need a pillar box)
'a right to wear hat' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to wear' is an insertion indicator (to wear something is to be covered up by it).
'hat' becomes 'pillbox' (pillbox is a kind of hat).
'a'+'r'='ar'
'ar' put inside 'pillbox' is 'PILLAR-BOX'.
'may be' acts as a link.
(Other definitions for pillar box that I've seen before include "Letter recepatacle" , "Red street furniture" , "Cylindrical letter collector" , "Where to put post" , "Mail receptacle" .)