A bloke runs riot on Circle Line (8)
I believe the answer is:
bakerloo
'line' is the definition.
(I've seen this before)
'a bloke runs riot on circle' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'riot' indicates an anagram.
'on' indicates putting letters inside (as in clothing 'on' a person).
'circle' becomes 'o' (O is a circle).
'bloke'+'r'='bloker'
'bloker' is an anagram of 'bkerlo'.
'a' put inside 'bkerlo' is 'bakerlo'.
'bakerlo'+'o'='BAKERLOO'
(Another definition for bakerloo that I've seen is " London Underground line represented in brown".)