Iron horse's pulling power? (6,9)
I believe the answer is:
animal magnetism
'iron' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'horse's pulling power?' is the wordplay.
'horse's' becomes 'animal' (I can't explain this - if you can you should believe this answer much more).
'pulling power?' becomes 'magnetism' (I've seen this before).
'animal'+'magnetism'='ANIMAL MAGNETISM'
Can you help me to learn more?
(Other definitions for animal magnetism that I've seen before include "Sexual attraction" , "Physical attractiveness" , "Zoo attraction" , "Polar bears might exert such a pull" .)