What it is when there's a bite in the air (5)
I believe the answer is:
nippy
'what it is when' is the definition.
I can't judge whether this defines the answer.
'bite in the air' is the wordplay.
'bite' becomes 'nip' (nip is a kind of bite).
'in the air' becomes 'py' (I am not sure about this - if you are sure you should believe this answer much more).
'nip'+'py'='NIPPY'
'there's a' is the link.
Can you help me to learn more?
(Other definitions for nippy that I've seen before include "active" , "Slightly cold?" , "A bit cold?" , "Able to move quickly" , "Speedy - cold" .)