It is a harvest moon.

(I did make an error; at the time of posting, it should have been Autumn, not August. I just wasn't seeing it; persistent headache today. My sincere apologies.)

In the sense of Harvest Moon, "Harvest" is describing the noun of moon, so it's functioning as an adjective.

August light when round perplexes the possessing minister (7)

(should have been Autumn light when round perplexes the possessing minister (7))

DESCRIPTION:

Autumn light when round = Harvest  (Harvest moon = the full moon closest to autumnal equinox)

WORDPLAY

perplexes [anagram indicator]

possessing = hast

minister = Rev

   anagram of hast Rev = harvest

nfs, you would have got this had I not got myself into a bulldozer headache-induced tangle; would you please take the next round?