Corps' first detail deployed in fortress (7)
I believe the answer is:
citadel
'fortress' is the definition.
(thesaurus)
'corps' first detail deployed' is the wordplay.
'first' suggests taking the first letters.
'deployed' indicates an anagram.
The first letter of 'corps' is 'c'.
'detail' anagrammed gives 'itadel'.
'c'+'itadel'='CITADEL'
'in' acts as a link.
(Other definitions for citadel that I've seen before include "Fort, fastness" , "Keep - castle" , "Stronghold into which people could go for shelter during a battle" , "Urban fortress" , "place of refuge" .)