Prisoner writing in circumstances (7)
I believe the answer is:
context
'circumstances' is the definition.
(thesaurus)
'prisoner writing' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'writing' becomes 'text' (text is a kind of writing**).
'con'+'text'='CONTEXT'
'in' is the link.
(Other definitions for context that I've seen before include "Frame of reference" , "what they relate to" , "Surrounding circumstances" , "relevant conditions" , "Words surrounding a word or phrase that may fix its true meaning" .)