Hidden eastern prisoner tried to break outside (9)
I believe the answer is:
recondite
'hidden' is the definition.
(thesaurus)
'eastern prisoner tried to break outside' is the wordplay.
'eastern' becomes 'e'.
'prisoner' becomes 'con' (abbreviation for 'convict').
'to break' indicates an anagram.
'outside' indicates putting letters inside.
'tried' is an anagram of 'rdite'.
'e'+'con'='econ'
'econ' put inside 'rdite' is 'RECONDITE'.
(Other definitions for recondite that I've seen before include "Enigmatic" , "Direct one to be obscure or difficult" , "(Of knowledge) obscure" , "not widely known" , "Difficult to understand" .)