Resolute, revolutionary fanatics besieging a church (7)
I believe the answer is:
staunch
'resolute' is the definition.
(I've seen this before)
'revolutionary fanatics besieging a church' is the wordplay.
'revolutionary' says the letters should be written in reverse (letters revolve around).
'fanatics' becomes 'nuts' (nut can mean an enthusiast or fanatic).
'besieging' means one lot of letters goes inside another.
'church' becomes 'ch' (common abbreviation).
'nuts' enclosing 'a' is 'nuats'.
'nuats' backwards is 'staun'.
'staun'+'ch'='STAUNCH'
(Other definitions for staunch that I've seen before include "way to limit bloodshed" , "Reliable" , "Hun acts in firm and loyal way" , "solid" , "Very loyal; stop (a flow of blood)" .)