Greek letter, maybe tau’s included (4)
I believe the answer is:
beta
'greek letter' is the definition.
(letter from the Greek alphabet)
'maybe tau's included' is the wordplay.
'included' indicates the answer is hidden within the clue (I've seen 'including' mean this).
'BETA' is hidden within 'maybe taus'.
(Other definitions for beta that I've seen before include "less than perfect mark" , "second class grade?" , "Second-class" , "sort of test" , "Character between alpha and gamma" .)