Pentagon losing power prepared for duty (7)
I believe the answer is:
Here is my best explanation:
'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 "Ship's size or carrying capacity", "Measure of cargo space", "Carrying capacity of cargo ships", "Capacity of ship", "Weight of cargo - ship's carrying capacity", "Charge for carrying", "No agent (anag)" and "Cargo weight".)