Second letter of the Greek alphabet (4)
I believe the answer is:
beta
'second letter of the greek alphabet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for beta that I've seen before include "Character between alpha and gamma" , "Second Greek letter - beat that" , "2nd letter of Greek alphabet" , "Second Greek letter, B" , "software under test" .)