Bird colony in castle by eastern railway (7)
I believe the answer is:
rookery
'bird colony' is the definition.
(I've seen this before)
'castle by eastern railway' is the wordplay.
'castle' becomes 'rook' (synonyms).
'by' says to put letters next to each other.
'eastern' becomes 'e'.
'railway' becomes 'ry' (abbreviation for railway).
'rook'+'e'+'ry'='ROOKERY'
'in' is the link.
(Other definitions for rookery that I've seen before include "Nesting-place" , "breeding ground" , "Lofty breeding-place" , "Noisy bird colony" , "Community of birds" .)