Read but never post (4)
I believe the answer is:
lurk
'read but never post' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lurk that I've seen before include "Lie in ambush" , "View an online forum without contributing" , "Behave in a sneaky and secretive manner" , "Linger around furtively" , "Hang around in a secretive way" .)