Blanket (7)
'blanket' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for overlie that I've seen before include "Sit on top of" , "To cover up" , "Put in position superior to" , "with crushing effect" , "Create coincidence" .)
(Other definitions for fort that I've seen before include "Defensive building" , "Keep" , "& 22 US Army post" , "US gold reserves are at - Knox" , "Garrison" .)