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