Erratic cop gets involved with sadism (9)
I believe the answer is:
spasmodic
'erratic' is the definition.
(thesaurus)
'cop gets involved with sadism' is the wordplay.
'gets involved with' is an anagram indicator.
'cop' anagrammed with 'sadism' makes 'SPASMODIC'.
(Other definitions for spasmodic that I've seen before include "Occasional" , "Done in brief irregular bursts" , "fitfully" , "Done in short bursts" , "Occurring in irregular bursts" .)