Some keep signatures from a Greek letter (3)
I believe the answer is:
psi
'a greek letter' is the definition.
(letter from the Greek alphabet)
'some keep signatures from' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'PSI' is hidden within 'keep signatures from'.
(Other definitions for psi that I've seen before include "Pounds per square inch (abbr.); Greek letter" , "Pounds per sq. in. (abbr.)" , "Greek letter; pounds per square inch (abbrev.)" , "Twenty-third letter of Greek alphabet" , "letter from abroad" .)