Wear son out in harbour (4)
I believe the answer is:
port
'harbour' is the definition.
(I've seen this before)
'wear son out' is the wordplay.
'wear' becomes 'sport' (sport can mean to wear or display on one's person).
'son' becomes 's' (genealogical abbreviation for son).
'out' indicates named letters should be taken away.
'sport' with 's' taken out is 'PORT'.
'in' is the link.
(Other definitions for port that I've seen before include "Light enough to carry" , "Drink passed to the left" , "Sweet red wine for after dinner" , "E.g. five [SWANSEA]" , "Harbour; drink; opening" .)