Letter used in teaching Greek (3)
I believe the answer is:
chi
'letter' is the definition.
(I know that chi is a type of letter)
'in teaching greek' is the wordplay.
'in' indicates a hidden word.
'CHI' can be found hidden inside 'teaching greek'.
'used' is the link.
(Other definitions for chi that I've seen before include "One character abroad" , "foreign character" , "Letter written by European" , "T'ai -, martial art" , "character from Euripides?" .)