Mount a rail the wrong way round (4)
I believe the answer is:
arab
'mount' is the definition.
(Arab is a type of horse or 'mount')
'a rail the wrong way round' is the wordplay.
'rail' becomes 'bar' (I've seen this in other clues).
'the wrong way round' shows that the letters should be reversed in order.
'bar' written backwards gives 'rab'.
'a'+'rab'='ARAB'
(Other definitions for arab that I've seen before include "Someone from the Middle East or North Africa, perhaps" , "Saudi, e.g." , "Small, lively breed of horse" , "Fine horse breed" , "Saudi or Omani, perhaps" .)