Post-It's regularly showing trident shapes (4)
I believe the answer is:
psis
'trident shapes' is the definition.
(the greek letter)
'post-it's regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'post'+'its'='postits'
The alternate letters of 'postits' are 'PSIS'.
'showing' is the link.
(Other definitions for psis that I've seen before include "letters from abroad" , "letters from Greece" .)