Sea advanced to claim river and harbour (6)
I believe the answer is:
marina
'harbour' is the definition.
(marina is a type of harbour)
'sea advanced to claim river' is the wordplay.
'sea' becomes 'main' (synonyms).
'advanced' becomes 'a' (abbreviation).
'to claim' indicates putting letters inside (inserted letters are claimed or taken in).
'river' becomes 'r' (abbreviation for river).
'main'+'a'='maina'
'maina' enclosing 'r' is 'MARINA'.
'and' is the link.
(Other definitions for marina that I've seen before include "Docking facility for pleasure craft" , "Dock for yachts and cruisers" , "boating facility" , "Docking place for pleasure craft" , "Airman (anag)" .)