First sign of the Greeks (5)
I believe the answer is:
alpha
This might be a double definition.
'first' is the first definition.
(thesaurus)
'sign of the greeks' is the second definition.
'alpha' can be an answer for 'sign' (alpha is a kind of sign). I'm not sure about the 'of the greeks' bit.
Can you help me to learn more?
(Other definitions for alpha that I've seen before include "First, best quality" , "letter written in Greek" , "Greek vowel" , "star of utmost brilliance" , "starter" .)