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