Fortress — it’s captured by rogue with the Spanish (7)
I believe the answer is:
citadel
'fortress' is the definition.
(thesaurus)
'it's captured by rogue with the spanish' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues**).
'captured by' indicates putting letters inside.
'rogue' becomes 'cad' (I've seen this before).
'with' says to put letters next to each other.
'the spanish' becomes 'el'.
'it' going inside 'cad' is 'citad'.
'citad'+'el'='CITADEL'
(Other definitions for citadel that I've seen before include "where Salvationists meet" , "Urban stronghold - dialect (anag)" , "Structure in place to protect" , "Kremlin" , "Fortress -- dialect (anag)" .)