Stowing silver, 1,000kg carrying capacity (7)
I believe the answer is:
tonnage
'carrying capacity' is the definition.
(how much a ship can carry in tons)
'stowing silver 1000kg' is the wordplay.
'stowing' indicates putting letters inside (inserted letters are stowed away).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'1000kg' becomes 'tonne' (a tonne or metric ton is 1000 kilograms).
'ag' put inside 'tonne' is 'TONNAGE'.
(Other definitions for tonnage that I've seen before include "Charge for carrying" , "No agent (anag)" , "Measure of cargo space" , "Ship's size or carrying capacity" , "Capacity of ship" .)