Castle regularly ignored nearby dense slum area (7)
I believe the answer is:
'dense slum area' is the definition.
(rookery can mean slum housing)
'castle regularly ignored nearby' is the wordplay.
'castle' becomes 'rook' (synonyms).
'regularly ignored' indicates alternate letters (letters removed at regular intervals).
The alternating letters of 'nearby' are 'ery'.
(Other definitions for rookery that I've seen before include "Colony of penguins or seals" , "Seal, penguin, colony" , "Nesting-place" , "Breeding colony of birds, also of seals or turtles" , "Community of birds" .)