One working to pen ten strong animals (4)
I believe the answer is:
oxen
'strong animals' is the definition.
(I've seen this before)
'one working to pen ten' is the wordplay.
'working' indicates anagramming the letters.
'to pen' is an insertion indicator.
'ten' becomes 'X' (Roman numeral).
'one' anagrammed gives 'oen'.
'oen' enclosing 'x' is 'OXEN'.
(Other definitions for oxen that I've seen before include "Heavy cattle" , "Large bovine animals" , "Beef stock" , "Doctored bulls" , "Cows and bulls" .)