Letter resembling a pitchfork (3)
I believe the answer is:
psi
'letter resembling a pitchfork' 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 letter; pressure unit" , "foreign letter" , "letter from abroad" , "Pounds per sq. in. (abbr.)" , "Twenty-third letter of Greek alphabet" .)