A dim ponderous criminal's usual method (5,8)
I believe the answer is:
modus operandi
'usual method' is the definition.
'modus operandi' can be an answer for 'method' (I have seen 'Method of working' mean 'modus operandi' so perhaps 'method' could also mean 'modus operandi'). I am not certain of the 'usual' bit.
'a dim ponderous criminal's' is the wordplay.
'criminal's' indicates an anagram (I've seen 'criminal' mean this).
'a'+'dim'+'ponderous'='adimponderous'
'adimponderous' anagrammed gives 'MODUS OPERANDI'.
Can you help me to learn more?
(Other definitions for modus operandi that I've seen before include "MO in full" , "Method of working" , "this shows how it's done" , "Classically typical style" , "Particular way of doing something (Latin)" .)