It’s cold, I see, at mid-September (3)
I believe the answer is:
ice
'it's cold' is the definition.
(I've seen this before)
'i see at mid-september' is the wordplay.
'see' becomes 'c' (the word for the letter, according to Chambers**).
'at' is a charade indicator (letters next to each other).
'mid' means to look at the middle letters.
'september' becomes 'sep' ('Sep' can be a synonym of 'september').
The middle letter of 'sep' is 'e'.
'i'+'c'+'e'='ICE'
(Other definitions for ice that I've seen before include "Rocks on which drink is served?" , "Cool down" , "Diamond" , "dessert" , "Glaze" .)