Frost beginning to drive squirrel away (5)
I believe the answer is:
hoard
'squirrel away' is the definition.
(synonyms)
'frost beginning to drive' is the wordplay.
'frost' becomes 'hoar' (synonyms).
'beginning to' says to take the initial letters.
The first letter of 'drive' is 'd'.
'hoar'+'d'='HOARD'
(Other definitions for hoard that I've seen before include "accumulated wealth" , "One's savings" , "Amass and store privately" , "Secret store of valuables" , "keep in reserve" .)