To compartment number one, back goes the horse (8)
I believe the answer is:
stallion
'the horse' is the definition.
(I've seen this in another clue)
'to compartment number one back' is the wordplay.
'to compartment' becomes 'stall' (I've seen this in another clue).
'number' becomes 'no' (abbreviation).
'one' becomes 'i' (Roman numeral).
'back' says the letters should be written backwards.
'no'+'i'='noi'
'noi' back-to-front is 'ion'.
'stall'+'ion'='STALLION'
'goes' acts as a link.
(Other definitions for stallion that I've seen before include "Male steed" , "Uncastrated adult male horse" , "Mount" , "Stud" , "Mare's mate" .)