One is wrong about a number of beasts (4)
I believe the answer is:
oxen
'beasts' is the definition.
(I've seen this in another clue)
'one is wrong about a number' is the wordplay.
'is wrong' is an anagram indicator.
'about' is an insertion indicator.
'a number' becomes 'x' (X is an example).
'one' is an anagram of 'oen'.
'oen' placed around 'x' is 'OXEN'.
'of' is the link.
(Other definitions for oxen that I've seen before include "Bovine animals, in yoke" , "Lowers" , "Cows and bulls" , "Large bovine animals" , "Domesticated bovine draft animals" .)