Pentagon losing power prepared for duty (7)
I believe the answer is:
tonnage
'duty' is the definition.
(tonnage is a kind of duty)
'pentagon losing power prepared' is the wordplay.
'losing' indicates named letters should be taken away.
'power' becomes 'p'.
'prepared' indicates anagramming the letters.
'pentagon' with 'p' taken away is 'entagon'.
'entagon' with letters rearranged gives 'TONNAGE'.
'for' is the link.
(Other definitions for tonnage that I've seen before include "Charge for carrying" , "Capacity of ship" , "Ship's size or carrying capacity" , "Weight of cargo - ship's carrying capacity" , "No agent (anag)" .)