Writer's source? A stylish prison (8,3)
I believe the answer is:
fountain pen
'writer's' is the definition.
(something which writes)
'source? a stylish prison' is the wordplay.
'source?' becomes 'fount' (fount can mean a fountain or source).
'stylish' becomes 'in'.
'prison' becomes 'pen' (both can mean an enclosure).
'fount'+'a'+'in'+'pen'='FOUNTAIN-PEN'
(Other definitions for fountain pen that I've seen before include "Ink writing device" , "Writing implement" , "Writer" .)