Siren enraged outside church finds Frank (7)
I believe the answer is:
sincere
'frank' is the definition.
(frank can mean sincere or honest)
'siren enraged outside church' is the wordplay.
'enraged' indicates anagramming the letters.
'outside' indicates putting letters inside.
'church' becomes 'CE' (abbreviation for Church of England).
'siren' with letters rearranged gives 'sinre'.
'sinre' enclosing 'ce' is 'SINCERE'.
'finds' acts as a link.
(Other definitions for sincere that I've seen before include "devout" , "Honest, heartfelt" , "Showing genuine feelings" , "Free from deception" , "Lacking in deceit" .)