The beasts cross one out wrongly! (4)
I believe the answer is:
oxen
'the beasts' is the definition.
(I have seen 'Beasts of burden ' mean 'oxen' so perhaps 'beasts' could also mean 'oxen')
'cross one out wrongly' is the wordplay.
'cross' indicates anagramming the letters (I've seen 'crossed' mean this).
'out' means one lot of letters goes inside another.
'wrongly' becomes 'x' (I can't justify this - if you can you should give a lot more credence to this answer).
'one' anagrammed gives 'oen'.
'oen' going around 'x' is 'OXEN'.
(Other definitions for oxen that I've seen before include "(E.g. draught) cattle" , "Domesticated cattle" , "Bovine animals, in yoke" , "Large bovine animals" , "Steers" .)