Large animal turned on one after beginning to run hard (5)
I believe the answer is:
rhino
'large animal' is the definition.
(I've seen this before)
'turned on one after beginning to run hard' is the wordplay.
'turned' shows that the letters should be reversed in order.
'one' becomes 'i' (Roman numeral**).
'after' is a charade indicator (letters next to each other).
'beginning to' indicates taking the first letters.
The first letters of 'run hard' is 'rh'.
'on'+'i'='oni'
'oni' backwards is 'ino'.
'ino' put after 'rh' is 'RHINO'.
(Other definitions for rhino that I've seen before include "Powerful herbivorous odd-toed ungulate, informally" , "Hi, Ron, this animal is abbreviated" , "Horned pachyderm" , "money in the past" , "The necessary" .)