Back starting tests in rehab is a swimmer (6)
I believe the answer is:
bather
'swimmer' is the definition.
(I know that swimmer can be written as bather)
'back starting tests in rehab' is the wordplay.
'back' says the letters should be written in reverse.
'starting' indicates taking the first letters.
'in' means one lot of letters goes inside another.
The initial letter of 'tests' is 't'.
't' put inside 'rehab' is 'rehtab'.
'rehtab' written backwards gives 'BATHER'.
'is a' acts as a link.
(Other definitions for bather that I've seen before include "one getting into the drink" , "man getting soaked?" , "Seaside swimmer" , "One dipping in the briny?" .)