Trident-shaped letter (3)
I believe the answer is:
psi
'trident-shaped letter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for psi that I've seen before include "Greek character" , "Pounds per sq. in. (abbr.)" , "Pounds per square inch (abbr.); Greek letter" , "Greek letter; pressure unit" , "Twenty-third letter of Greek alphabet" .)