Brief and secular, prisoner included (7)
I believe the answer is:
laconic
'brief' is the definition.
(synonyms)
'secular prisoner included' is the wordplay.
'secular' becomes 'laic' (synonyms).
'prisoner' becomes 'con' (abbreviation for 'convict').
'included' indicates putting letters inside.
'laic' going around 'con' is 'LACONIC'.
'and' acts as a link.
(Other definitions for laconic that I've seen before include "Col Cain (anag) - expressed in a few words" , "Conical (anag) - pithy" , "without verbosity" , "without many words" , "Spartan" .)