Damage in American harbour (6)
I believe the answer is:
marina
'harbour' is the definition.
(I've seen this before)
'damage in american' is the wordplay.
'damage' becomes 'mar' (to mar is to spoil or damage).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'mar'+'in'+'a'='MARINA'
(Other definitions for marina that I've seen before include "where yachts moor" , "part of harbour?" , "Pleasure-boat dock" , "Dock for yachts and cruisers" , "seaside area" .)