Character transfixed by horse's carrying capacity (7)
I believe the answer is:
tonnage
'carrying capacity' is the definition.
(how much a ship can carry in tons)
'character transfixed by horse's' is the wordplay.
'character' becomes 'tone' (tone can mean a character or mood).
'transfixed by' means one lot of letters goes inside another.
'horse' becomes 'nag' (term for an old horse).
'tone' placed around 'nag' is 'TONNAGE'.
(Other definitions for tonnage that I've seen before include "No agent (anag)" , "Capacity of ship" , "Measure of cargo space" , "Charge for carrying" , "Weight of cargo - ship's carrying capacity" .)