Peckham wide boy knowing where Dover is (8)
I believe the answer is:
delaware
'where dover is' is the definition.
(Dover is the capital of Delaware)
'peckham wide boy knowing' is the wordplay.
'peckham wide boy' becomes 'Del' (Delboy Trotter from sitcom Only Fools and Horses).
'knowing' becomes 'aware' (I've seen this before).
'del'+'aware'='DELAWARE'
(Other definitions for delaware that I've seen before include "Small E. coast US state" , "The First State (to ratify the US constitution)" , "American state, capital Dover" , "NE US state" , "State of northeast US" .)