Horse from stable popular round ring (8)
I believe the answer is:
'horse' is the definition.
'stable popular round ring' is the wordplay.
'stable' becomes 'stall' (I've seen this in other clues).
'popular' becomes 'in'.
'round' means one lot of letters goes inside another.
'ring' becomes 'o'.
'in' enclosing 'o' is 'ion'.
'from' is the link.
(Other definitions for stallion that I've seen before include "Male horse, often used for breeding" , "Male steed" , "Mare's mate" , "Male horse kept for breeding" , "Male horse lost in atolls" .)