A lot of icy weather beginning to penetrate nap (5)
I believe the answer is:
sleep
'nap' is the definition.
(I know that nap can be written as sleep)
'a lot of icy weather beginning to penetrate' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'icy weather' becomes 'sleet' (I've seen this in another clue).
'beginning to' says to take the initial letters.
The initial letter of 'penetrate' is 'p'.
'sleet' with its final letter taken away is 'slee'.
'slee'+'p'='SLEEP'
(Other definitions for sleep that I've seen before include "Drop off" , "Natural state of unconscious rest" , "''... that knits up the ravell'd sleeve of care'' (Macbeth)" , "crash" , "Shut-eye" .)