Apply as a swimmer (3)
I believe the answer is:
dab
This clue is a double definition.
'apply' is the first definition.
(dabbing is a kind of applying)
'a swimmer' is the second definition.
(I've seen this before)
'as' is the link.
(Other definitions for dab that I've seen before include "Small North Atlantic flatfish" , "[SOMETHING FISHY]" , "Press gently; flatfish" , "Apply light touches of colour to the fish" , "Flounder" .)