Gammas

7 clues
Greek letters
Deltas' preceders
Some Greek letters
Followers of betas
More Greek letters
Third items in series
Ultrahigh-energy photons
Similar answers:
Abomas
Achristmascarlo
Achristmascarob
Achristmascarol
Achristmasclaro
Achristmascoral
Achristmasstory
Achristmastale
Ademas
Admass
Adoubtingthomas
Afterchristmasandall
Aftermash
Aftermass
Agam
Agama
Agamas
Agamblernamedjbmcnair
Agame
Agameandfinished