Write from prison (3)
I believe the answer is:
pen
I believe this clue is a double definition.
'write' is the first definition.
(I know that write can be written as pen)
'prison' is the second definition.
(both can mean an enclosure)
'from' acts as a link.
(Other definitions for pen that I've seen before include "International Association of poets, playwrights etc" , "Cage - bird" , "Coop" , "See 20" , "Writing tool" .)