Enclosures for female swans (4)
I believe this is a double definition.
'enclosures' is the first definition.
(pen is a kind of enclosure)
'female swans' is the second definition.
(I've seen this before)
'for' acts as a link.
(Other definitions for pens that I've seen before include "birds", "Female swans", "Literary thought or reflection", "writers", "Writing implements".)