System of measuring crime developed around middle of century (6)
I believe the answer is:
metric
'system of measuring' is the definition.
'crime developed around middle of century' is the wordplay.
'developed' indicates anagramming the letters.
'around' indicates putting letters inside.
'middle of' means to look at the middle letters.
The middle letter of 'century' is 't'.
'crime' anagrammed gives 'meric'.
'meric' enclosing 't' is 'METRIC'.
(Other definitions for metric that I've seen before include "Like some feet" , "Like decimal measuring system" , "Non-Imperial system of measurement" , "Consisting of verses; pertaining to one system of measurement" , "Relating to poetic rhythm" .)