Present sentry's first and last dissent (6)
I believe the answer is:
heresy
'dissent' is the definition.
(thesaurus)
'present sentry's first and last' is the wordplay.
'present' becomes 'here' (both can mean in attendance).
'first and last' means to remove the middle letters (only the first and last letters are used).
'sentry' with its middle removed is 'sy'.
'here'+'sy'='HERESY'
(Other definitions for heresy that I've seen before include "Opinion opposed to conventional belief" , "Religious non-conformity" , "Doctrine that differs from the orthodox position" , "not in good faith" , "Unorthodoxy - apostasy" .)