Mistrust religious school detaining us with pressure (7)
I believe the answer is:
suspect
'mistrust' is the definition.
('suspect' can be a synonym of 'mistrust')
'religious school detaining us with pressure' is the wordplay.
'religious school' becomes 'sect'.
'detaining' is an insertion indicator.
'with' means one lot of letters go next to another.
'pressure' becomes 'p' (abbreviation).
'us'+'p'='usp'
'sect' placed around 'usp' is 'SUSPECT'.
(Other definitions for suspect that I've seen before include "Believe (something) possible or probable" , "Imagine to be true" , "One believed to have committed a crime" , "Doubt; mistrust" , "smell a rat produces" .)