Writer in an American prison (3)
I believe the answer is:
pen
This clue is a double definition.
'writer' is the first definition.
(something which writes)
'an american prison' is the second definition.
(short for penitentiary)
'in' acts as a link.
(Other definitions for pen that I've seen before include "Female swan" , "International Association of poets, playwrights etc" , "American jug?" , "Writing implement" , "See 20" .)