Accumulation of frost by day (5)
I believe the answer is:
hoard
'accumulation' is the definition.
(hoard is a kind of accumulation)
'frost by day' is the wordplay.
'frost' becomes 'hoar' ('hoar' can be a synonym of 'frost'**).
'by' means one lot of letters go next to another.
'day' becomes 'd'.
'hoar'+'d'='HOARD'
'of' acts as a link.
(Other definitions for hoard that I've seen before include "Store of treasure" , "Accumulated store" , "Rhoda (anag.)" , "Valuable store" , "Store of money etc, often hidden" .)