Wind Harry Potter's friend up before festival (3-6)
I believe the answer is:
nor-easter
'wind' is the definition.
(I have seen 'Cold wind' mean 'nor easter' so perhaps 'wind' could also mean 'nor easter')
'harry potter's friend up before festival' is the wordplay.
'harry potter's friend' becomes 'ron' (I've seen this in another clue).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'before' means one lot of letters go next to another.
'festival' becomes 'easter' (Christian festival).
'ron' back-to-front is 'nor'.
'nor'+'easter'='NOR-EASTER'
(Another definition for nor-easter that I've seen is " winds?".)