See enigma's first code is cracked (7)
I believe the answer is:
diocese
'see' is the definition.
(both can mean a bishop's associated area)
'enigma's first code is cracked' is the wordplay.
'enigma's first' becomes 'e' (1st letter of 'enigma').
'cracked' indicates an anagram.
'e'+'code'+'is'='ecodeis'
'ecodeis' with letters rearranged gives 'DIOCESE'.
(Other definitions for diocese that I've seen before include "District under care of bishop" , "Bishop's responsibility" , "Bishop's area" , "District under a bishop's authority" , "Area under bishop" .)