This curtailed each criminal's activity (8)
I believe the answer is:
thievery
'criminal's activity' is the definition.
'thievery' can be an answer for 'activity' (thievery is a kind of activity). I am not sure about the 'criminal's' bit.
'this curtailed each' is the wordplay.
'curtailed' means to remove the last letter.
'each' becomes 'every' (synonyms).
'this' with its last letter taken away is 'thi'.
'thi'+'every'='THIEVERY'
Can you help me to learn more?
(Other definitions for thievery that I've seen before include "wrong-doing" , "Knocking off" , "Act of stealing" , "Practice of stealing" , "criminal activity" .)