Many steal about a stronghold (6)
I believe the answer is:
castle
'a stronghold' is the definition.
(I know that castle is a type of citadel)
'many steal about' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'about' indicates anagramming the letters.
'steal' with letters rearranged gives 'astle'.
'c'+'astle'='CASTLE'
(Other definitions for castle that I've seen before include "Citadel" , "Imposing residence, perhaps fortified" , "Move rook and king in the same turn" , "Rook, on chess board" , "Another word for chess rook" .)