Earth’s satellite (4)
(Other definitions for boon that I've seen before include "Blessing, favour" , "Favour, request" , "Godsend" , "Something beneficial - a blessing" , "Benefaction" .)
'earth's satellite' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for moon that I've seen before include "Remove boxers, maybe" , "who headed a religious 29 [CULT]" , "Idle (around) - satellite" , "Phoebe" , "Daydream - Europa of Jupiter, say" .)