Sixth Greek letter (4)
I believe the answer is:
zeta
'sixth greek letter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for zeta that I've seen before include "Sixth letter of the Greek alphabet" , "Spartan character" , "Welsh actress, Catherine __-Jones, b. 1969" , "Greek character" , "Greek letter (for Catherine Jones?)" .)