Number seen running around London area in a state (9)
I believe the answer is:
tennessee
'a state' is the definition.
(I know that Tennessee is a U.S. state)
'number seen running around london area' is the wordplay.
'number' becomes 'ten' (ten is a kind of whole number).
'running' indicates anagramming the letters (the letters run around to different places).
'around' is an insertion indicator.
'london area' becomes 'SE' (postcode for south east London).
'seen' is an anagram of 'nese'.
'nese' placed around 'se' is 'nessee'.
'ten'+'nessee'='TENNESSEE'
'in' acts as a link.
(Other definitions for tennessee that I've seen before include "... Williams, and southeastern US state" , "Williams, say" , "Seen teens in this US state" , "Famous American playwright . . . . . . . . . Williams" , "Nashville's state" .)