The beasts cross one out, possibly (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 possibly' is the wordplay.
'cross' indicates anagramming the letters (a cross or irritated version of the letters).
'out' is an insertion indicator.
'possibly' becomes 'x' (I can't justify this - if you can you should give a lot more credence to this answer).
'one' is an anagram of 'oen'.
'oen' enclosing 'x' is 'OXEN'.
Can you help me to learn more?
(Other definitions for oxen that I've seen before include "Large bovine animals" , "Beasts of burden or domestic cattle" , "Doctored bulls" , "Draught animals" , "strong animals" .)