Duty to get horse into physical condition (7)
I believe the answer is:
tonnage
'duty' is the definition.
(I know that tonnage is a type of duty)
'horse into physical condition' is the wordplay.
'horse' becomes 'nag' (term for an old horse).
'into' is an insertion indicator.
'physical condition' becomes 'tone' (tone is a kind of condition. I am not sure about the 'physical' bit.).
'nag' going within 'tone' is 'TONNAGE'.
'to get' is the link.
Can you help me to learn more?
(Other definitions for tonnage that I've seen before include "Charge for carrying" , "Weight of cargo - ship's carrying capacity" , "Capacity of ship" , "Ship's size or carrying capacity" , "No agent (anag)" .)