Rocking robin, huge one seen next door (9)
I believe the answer is:
Here is my best explanation:
'one seen next door' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'rocking robin huge' is the subsidiary indication.
'rocking' indicates an anagram.
'robinhuge' with letters rearranged gives 'neighbour'.
(Other definitions for neighbour that I've seen before include "possibly providing part of watch?", "Someone in the street", "Person living close by", "Be next to", "someone close", "Person who lives close by", "Person alongside", "Fellow human", "One lives nearby" and "lovable person?".)