The fellow left to go down to island capital (8)
I believe the answer is:
helsinki
'capital' is the definition.
(capital of Finland)
'the fellow left to go down to island' is the wordplay.
'the fellow' becomes 'he' ('he' refers to a man).
'left' becomes 'l' (common abbreviation).
'to go down' becomes 'sink' (sinking is a kind of going down).
'to island' becomes 'i' (geographical abbreviation).
'he'+'l'+'sink'+'i'='HELSINKI'
(Other definitions for helsinki that I've seen before include "Finnish capital" , "1952 Olympic Games venue" , "Capital of Finland" , "Is he link to a European city?" , "port" .)