Horse that's first back to its compartment (8)
I believe the answer is:
stallion
'horse' is the definition.
(I know that stallion is a type of horse)
'first back to its compartment' is the wordplay.
I cannot quite see how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is present in the answer.
'compartment' could be 'stall' (I have seen 'Stable compartment ' mean 'stall' so perhaps 'compartment' could also mean 'stall') and 'stall' is found within the answer.
The remaining letters 'on' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for stallion that I've seen before include "Male steed" , "Stud" , "Mare's mate" , "Male horse, often used for breeding" , "Uncastrated adult male horse" .)