Writer’s enclosure (3)
I believe the answer is:
pen
I believe this is a double definition.
'writer's' is the first definition.
(I've seen this before)
'enclosure' is the second definition.
(pen is a kind of enclosure)
(Other definitions for pen that I've seen before include "International Association of poets, playwrights etc" , "Small enclosure" , "'The ... is mightier than the sword'" , "Writing instrument" , "Enclosure; a swan" .)