Crossword Genius

Post-It's regularly showing trident shapes (4)

Ross

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" .)

I've seen this clue in The Everyman puzzle in The Observer.
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