One in centre of Naples, sullen traveller (7)
I believe the answer is:
pilgrim
'traveller' is the definition.
(pilgrim is a kind of traveller)
'one in centre of naples sullen' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'centre of' says to take the centre.
'sullen' becomes 'grim'.
The centre of 'naples' is 'pl'.
'i' inserted within 'pl' is 'pil'.
'pil'+'grim'='PILGRIM'
(Other definitions for pilgrim that I've seen before include "these early Fathers" , "Palmer" , "Traveller to a holy place" , "One of those who sailed to New England in the Mayflower, 1620" , "... emigrants to Massachusetts" .)