Stable one working horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(thesaurus)
'stable one working' is the wordplay.
'stable' becomes 'stall' (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'working' becomes 'on'.
'stall'+'i'+'on'='STALLION'
(Other definitions for stallion that I've seen before include "Uncastrated adult male horse" , "Male horse lost in atolls" , "Stud" , "Male horse kept for breeding" , "Mare's mate" .)