Worst thing, this pet running wild (3,4)
I believe the answer is:
the pits
'worst thing' is the definition.
'this pet running wild' is the wordplay.
'running wild' indicates an anagram.
'this'+'pet'='thispet'
'thispet' anagrammed gives 'THE PITS'.
(Other definitions for the pits that I've seen before include "pants" , "Absolutely worst person possible" , "A very bad situation (informal)" , "place to service a car?" , "Those places offering a quick tyre change" .)