1 He, coming home at undern-time, there found
undern-time > {The third hour of the day, i.e. about 9 a.m.; noon; the afternoon or evening. Hamilton (1980) suggests that "noon" makes good contextual sense, since the witch's son finds Florimell as dazzling as "the bright Sunne"}
2 The fairest creature that he ever saw, 3 Sitting beside his mother on the ground; 4 The sight whereof did greatly him adaw,
whereof > of whom adaw > confound, daunt, daze (from OFr adanter: cf. SC, gloss to "Februarie"); also, on a second reading of Cantos vii and viii: awaken (cognate with "dawn") whence: arouse (i.e. sexually. "Daw" (aphetic for "jackdaw") in early Scottish and northern dialect also means "simpleton" (cf. Hubberd 913); thus Spenser might have intended that the verb "to adaw" should also connote "to dumbfound")
5 And his base thought with terror and with awe 6 So inly smote that, as one who had gazed
inly > inwardly
7 On the bright sun unwares does soon withdraw
unwares > unwittingly
8 His feeble eyes, with too much brightness dazed, 9 So stared he on her, and stood long while amazed.
So > (The parenthesis causes a problem, but the meaning is clear)