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 subsidiary indication.
'losing' suggests deleting specific letters.
'power' becomes 'p'.
'prepared' is an anagram indicator.
'pentagon' with 'p' taken away is 'entagon'.
'entagon' is an anagram of '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", "Capacity of ship", "Cargo weight", "Weight of cargo - ship's carrying capacity", "No agent (anag)" and "Carrying capacity of cargo ships".)