Guess a second letter (4)
I believe the answer is:
beta
'second letter' is the definition.
(I've seen this in another clue)
'guess a' is the wordplay.
'guess' becomes 'bet' (I can't explain this - if you can you should give a lot more credence to this answer).
'bet'+'a'='BETA'
Can you help me to learn more?
(Other definitions for beta that I've seen before include "Second-class" , "Second Greek letter, B" , "Character between alpha and gamma" , "software under test" , "character in Marathon" .)