Old American swimmer can be uncouth (6)
I believe the answer is:
oafish
'be uncouth' is the definition.
(I've seen this before)
'old american swimmer' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'swimmer' becomes 'fish' (fish swim).
'o'+'a'+'fish'='OAFISH'
'can' acts as a link.
(Other definitions for oafish that I've seen before include "Like a boorish person" , "Uncouth" , "Loutish" , "Doltish" , "lacking elegance?" .)