Castle tower (7)
(Other definitions for bastion that I've seen before include "Obtains in fortified place or stronghold" , "fortress" , "Obtains (anag.)" , "part of stronghold" , "Projecting part of a fortification" .)
'castle tower' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for turret that I've seen before include "Small corner tower" , "Little (e.g. castle) tower" , "Keep feature" , "Small tower on defensive wall - or tank" , "Small tower on building or tank" .)