Hold like rich garners the full ripen'd grain (4)
Words turn into produce in this metaphor, a nice riff on the whole question of the value (or the productivity) of literature, which has been bugging writers since, well, the beginning of time. Flour, one of the most basic of food staples, is made from wheat. In one stroke, Keats makes language as necessary as the food we eat.
the night's starr'd face, (5)
Keats personifies the night, turning it into a figure with which he can interact. Ironically, this "face" is described in more detail than the actual "fair creature" he mentions later.
Huge cloudy symbols of a high romance, (6)
Where should you look to find signs of love? Why, in the sky, of course! Keats seems to be fashioning nature into a natural extension of his own emotional register. When he's happy and in love, the clouds bear the message up in the sky.