The second letter of the Greek alphabet (4)
I believe the answer is:
beta
'the second letter of the greek alphabet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for beta that I've seen before include "less than perfect mark" , "Second-rate" , "Second-class" , "Character between alpha and gamma" , "Second letter of the Greek alphabet" .)