Frost's old-fashioned poem (4)
I believe the answer is:
rime
I believe this is a double definition.
'frost's' is the first definition.
(rime is frost by freezing water vapour)
'old-fashioned poem' is the second definition.
(archaic spelling of 'rhyme')
(Other definitions for rime that I've seen before include "Hoar frost" , "Hoarfrost" , "Ice" , "Type of frost" , "Ancient Mariner produced?" .)