Enclosures for swans (4)
I believe the answer is:
pens
I believe this clue is a double definition.
'enclosures' is the first definition.
(pen is a kind of enclosure)
'for swans' is the second definition.
(pen is a kind of swan)
(Other definitions for pens that I've seen before include "Literary thought or reflection" , "Writing implements" , "birds" , "stalls" , "Livestock enclosures" .)