Eighth letter in the Greek alphabet (5)
I believe the answer is:
theta
'eighth letter in the greek alphabet' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for theta that I've seen before include "8th letter of the Greek alphabet" , "letter from abroad" , "Greek letter used in trigonometry" , "Make amends for" , "Greek letter equivalent to th" .)