Again question defensive structure (7)
I believe the answer is:
redoubt
'defensive structure' is the definition.
(redoubt is a kind of defensive structure)
'again question' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'question' becomes 'doubt' (synonyms).
're'+'doubt'='REDOUBT'
(Other definitions for redoubt that I've seen before include "Keep losing confidence in" , "Fortified stronghold" , "Defensive fieldwork" , "Military defence-work" , "21 ac fortification" .)