Animal in one pen being mounted (5)
I believe the answer is:
oribi
'animal' is the definition.
(type of antelope)
'one pen being mounted' is the wordplay.
'one' becomes 'i' (Roman numeral).
'pen' becomes 'biro' (Biro is an example).
'being mounted' shows that the letters should be reversed in order.
'i'+'biro'='ibiro'
'ibiro' backwards is 'ORIBI'.
'in' is the link.
(Other definitions for oribi that I've seen before include "Small African antelope" , "small hoofed beast" , "Beastly sort" , "resident of Savannah" , "type of antelope" .)