Of these beasts, the wild one is about to get cross! (4)
I believe the answer is:
oxen
'of these beasts the wild' is the definition.
The definition suggests an adverb but the answer is not.
'one is about to get cross' is the wordplay.
'is about' indicates anagramming the letters.
'to get' means one lot of letters goes inside another.
'cross' becomes 'x' (X is cross-shaped).
'one' with letters rearranged gives 'oen'.
'oen' enclosing 'x' is 'OXEN'.
Can you help me to learn more?
(Other definitions for oxen that I've seen before include "(E.g. draught) cattle" , "Neat types" , "Draught animals" , "Steers" , "Large bovine animals" .)