Because, at the revolutionary centre, there's a stronghold (4)
I believe the answer is:
fort
'a stronghold' is the definition.
(I've seen this in another clue)
'because at the revolutionary centre' is the wordplay.
'because' becomes 'for' (I've seen this in another clue).
'at' says to put letters next to each other.
'centre' indicates the central letters.
The central letter of 'revolutionary' is 't'.
'for'+'t'='FORT'
'there's' acts as a link.
(Other definitions for fort that I've seen before include "Defensive stronghold" , "Keep" , "& 4 Down Army post in USA" , "Strategic position" , "US gold reserves are at - Knox" .)