A writer in prison (3)
I believe the answer is:
pen
I believe this is a double definition.
'a writer' is the first definition.
(something which writes)
'prison' is the second definition.
(both can mean an enclosure)
'in' is the link.
(Other definitions for pen that I've seen before include "Writing implement" , "Cage - bird" , "International Association of poets, playwrights etc" , "Enclosure; a swan" , "Confine the writer" .)