Tool used on metal horse (spotted) (7)
I believe the answer is:
hacksaw
'tool' is the definition.
(hacksaw is a kind of tool)
'on metal horse spotted' is the wordplay.
'on' is a charade indicator (letters next to each other).
'metal horse' becomes 'hack' (hack is a kind of horse. I am not sure about the 'metal' bit.).
'spotted' becomes 'saw' (spot can mean to see).
'hack' next to 'saw' is 'HACKSAW'.
'used' acts as a link.
Can you help me to learn more?
(Other definitions for hacksaw that I've seen before include "means of cutting lead article?" , "Metal cutter" , "Blade in a frame under tension" , "Toothed tool for cutting metal" , "One cuts" .)