G32 ἄγγελος aggelos; to bring tidings; a messenger; especially an “angel”; by implication a pastor: - angel, messenger.
Those saying the "angel" was a messenger are correct, even if they did come to it sideways. The messenger can be a man, a heavenly being, and even Jesus (who is God) Himself appeard as a messenger/angel to Abraham at the Oaks of Mamre (along with 2 heavenly messengers/"angels").
Since this was a vision of heaven it could have been a dead prophet. Not sure about that though. It's a vision so the dead prophet could have been just been an image in his mind.
And by the way, the term typically translated "angel" in the OT is "malach," which means the same; "messenger."