One has to be ten to get an animal (4)
I believe the answer is:
ibex
'an animal' is the definition.
(I've seen this before)
'one has to be ten' is the wordplay.
'one' becomes 'i' (Roman numeral).
'has' means one lot of letters go next to another.
'ten' becomes 'X' (Roman numeral).
'i'+'be'+'x'='IBEX'
'to get' acts as a link.
(Other definitions for ibex that I've seen before include "being nimble-footed" , "Wild goat with large recurved horns" , "Wild mountain goat" , "Sure-footed climber" , "Sure-footed type" .)