It's heavenly in grammar schools (4)
I believe the answer is:
mars
'it's heavenly' is the definition.
I can't tell whether this definition defines the answer.
'in grammar schools' is the wordplay.
'in' says the answer is hidden in the clue.
'MARS' is hidden within 'grammar schools'.
Can you help me to learn more?
(Other definitions for mars that I've seen before include "part of suite" , "RS [RED STAR] (not technically)" , "War god - chocolate bar - spoils" , "Holst's Bringer of War" , "Ruins" .)