A danger to swimmers in Oregon and California (4)
I believe the answer is:
orca
'a danger to swimmers' is the definition.
(killer whale)
'oregon and california' is the wordplay.
'oregon' becomes 'or'.
'and' means one lot of letters go next to another.
'california' becomes 'ca'.
'or'+'ca'='ORCA'
'in' is the link.
(Other definitions for orca that I've seen before include "Massive killer" , "swimmer in black and white" , "Danger for swimmers" , "Large mammal" , "27 [WHALE]" .)