A lock followed for the stronghold (8)
I believe the answer is:
fortress
'stronghold' is the definition.
(I know that fortress is a type of citadel)
'a lock followed for the' is the wordplay.
'a lock' becomes 'ress' (I've seen this in another clue).
'followed' means one lot of letters go next to another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'for'+'t'='fort'
'ress' put after 'fort' is 'FORTRESS'.
(Other definitions for fortress that I've seen before include "Place that's secure" , "it's hard to break into" , "Military stronghold" , "Secure building" , "Strong point" .)