Telepathic letters (3)
I believe the answer is:
'telepathic letters' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for esp that I've seen before include "Apparent power to perceive things that others can't, initially" , "Apparent power to perceive things that others can't, in short" , "Second sight (abbr)" , "Extrasensory perception (abbrev.)" , "Initially, clairvoyance or telepathy" .)