All right, assistant returned with one African ruminant (5)
I believe the answer is:
okapi
'african ruminant' is the definition.
(I've seen this in another clue)
'all right assistant returned with one' is the wordplay.
'all right' becomes 'ok' (synonyms).
'assistant' becomes 'PA' (personal assistant).
'returned' shows that the letters should be reversed in order.
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'pa' reversed gives 'ap'.
'ok'+'ap'+'i'='OKAPI'
(Other definitions for okapi that I've seen before include "Central African resident" , "Wild African" , "Giraffe-like mammal" , "Ruminant related to the giraffe" , "creature" .)