Vast moor with hen flying — a feature of autumn night sky? (7,4)
I believe the answer is:
harvest moon
'a feature of autumn night sky?' is the definition.
(term for the September full moon)
'vast moor with hen flying' is the wordplay.
'with' is a charade indicator (letters next to each other).
'flying' indicates anagramming the letters (letters fly around into new positions).
'vast'+'moor'='vastmoor'
'vastmoor' after 'hen' is 'henvastmoor'.
'henvastmoor' with letters rearranged gives 'HARVEST MOON'.
(Other definitions for harvest moon that I've seen before include "Lunar appearance closest to autumn equinox" , "One may appear in September" , "It's seen close to the autumn equinox" , "Regular spectacle overhead" , "Mid-September occurrence in the night sky" .)