My word! One's performing wild antics (8)
I believe the answer is:
rowdyism
'wild antics' is the definition.
(wild or rowdy behaviour)
'my word one's performing' is the wordplay.
I cannot really see how this works, but
an anagram of 'word' is 'rowd' which is located in the answer.
'one's' could be 'is' (Roman numeral) and 'is' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for rowdyism that I've seen before include "Disorderliness" , "yobbish behaviour" , "bad behaviour" , "Noisy and disorderly behaviour" , "Unruly behaviour" .)