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' says to put letters next to each other.
'mid' says to take the centre (mid- can mean the middle of something).
'september' becomes 'sep' (short for September).
The middle of 'sep' is 'e'.
'i'+'c'+'e'='ICE'
(Other definitions for ice that I've seen before include "Solid water" , "If it's thin, skating on it can be dangerous" , "Diamonds (informal)" , "Girl's best friends" , "To put in a drink or slide on" .)