Tower of strength! (4)
'tower of strength' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for fort that I've seen before include "Defensive stronghold" , "castle, perhaps" , "US gold reserves are at - Knox" , "William in Scotland" , "Keep" .)
(Other definitions for tug that I've seen before include "Powerful boat" , "A sharp pull or a towing boat" , "jerk" , "Pull forcibly" , "Boat to pull larger ones" .)
(Other definitions for anchor that I've seen before include "Holding device for ship" , "It holds the ship in place" , "Firmly secure; moor" , "Ship-mooring device" , "TV journalist" .)