When kids knock, greeting tiny one in the middle (9)
I believe the answer is:
halloween
'when kids knock' is the definition.
(children knock on doors at Halloween)
'greeting tiny one in the middle' is the wordplay.
'greeting' becomes 'hallo' (alternative to hello).
'tiny' becomes 'wee' (I've seen this before).
'in the middle' says to take the centre.
The centre of 'one' is 'n'.
'hallo'+'wee'+'n'='HALLOWEEN'
(Other definitions for halloween that I've seen before include "Evening of masquerade" , "special night" , "Light-in-pumpkin festival" , "Night of October thirty-first" , "Trick or treat day" .)