Weight has horse in mood (7)
I believe the answer is:
tonnage
'weight' is the definition.
(I've seen this before)
'horse in mood' is the wordplay.
'horse' becomes 'nag' (term for an old horse).
'in' means one lot of letters goes inside another.
'mood' becomes 'tone' (tone is a kind of mood).
'nag' put inside 'tone' is 'TONNAGE'.
'has' acts as a link.
(Other definitions for tonnage that I've seen before include "Measure of cargo space" , "Ship's size or carrying capacity" , "No agent (anag)" , "Capacity of ship" , "Weight of cargo - ship's carrying capacity" .)