Tactful cop with mass in Capitol running riot (10)
I believe the answer is:
diplomatic
'tactful' is the definition.
(similar in meaning)
'cop with mass in capitol running riot' is the wordplay.
I cannot quite see how this works, but
'cop' could be 'di' (abbreviation for Detective Inspector) and 'di' is found in the answer.
'mass' could be 'm' (symbol used in Physics e.g. f=ma) and 'm' is found in the answer.
an anagram of 'capitol' is 'ploatic' which is located in the left over letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for diplomatic that I've seen before include "Tactful" , "Discreet" .)