Engineers abandoning a derelict broken-down castle? (7)
I believe the answer is:
citadel
'castle?' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for citadel that I've seen before include "Keep - castle" , "Stronghold usually within the walls" , "Stronghold into which people could go for shelter during a battle" , "Bastion" , "Kremlin" .)