Because, at the revolutionary centre, there’s a stronghold (4)
I believe the answer is:
fort
'a stronghold' is the definition.
(thesaurus)
'because at the revolutionary centre' is the wordplay.
'because' becomes 'for' (I've seen this before).
'at' means one lot of letters go next to another.
'centre' indicates the central letters.
The middle letter of 'revolutionary' is 't'.
'for'+'t'='FORT'
'there's' is the 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" .)