Letter between eta and iota (5)
I believe the answer is:
theta
'letter between eta and iota' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for theta that I've seen before include "8th letter of the Greek alphabet" , "Foreign letter" , "Letter to the Corinthians" , "Greek letter used in trigonometry" , "Character from overseas" .)