A swimmer in the pink (6)
I believe the answer is:
salmon
I believe this clue is a double definition.
'a swimmer' is the first definition.
(salmon is a kind of aquatic vertebrate)
'pink' is the second definition.
(salmon pink is a kind of pink)
'in the' is the link.
(Other definitions for salmon that I've seen before include "In Gaelic myth, the fish of wisdom" , "Fish (often smoked)" , "Pale pinkish orange colour" , "Food fish; pink shade" , "Oddly, Mal's no fish" .)