Either end of Milton's poetic early period (4)
I believe the answer is:
morn
'poetic early period' is the definition.
(poetic or archaic term for the morning)
'either end of milton's' is the wordplay.
'either' becomes 'mor' (I can't explain this - if you can you should give a lot more credence to this answer).
'end of' suggests the final letters.
The last letter of 'milton' is 'n'.
'mor'+'n'='MORN'
(Other definitions for morn that I've seen before include "PM? No" , "Day's first part(poetic)" , "Early part of day (poetic)" , "First part of day (poet.)" .)