Capital journey round Long Island (7)
I believe the answer is:
tripoli
'capital' is the definition.
(capital of Libya)
'journey round long island' is the wordplay.
'journey' becomes 'trip' (trip is a kind of journey).
'round' becomes 'o' (round can mean a round object).
'long island' becomes 'LI' (abbreviation).
'trip'+'o'+'li'='TRIPOLI'
(Other definitions for tripoli that I've seen before include "Libyan city" , "North African city" , "Capital of Libya" , "powdery deposit" , "North African capital" .)