Goes for a swim tentatively to start rehab (6)
I believe the answer is:
bather
'goes for a swim' is the definition.
I can't tell whether this definition defines the answer.
'tentatively to start rehab' is the wordplay.
'tentatively' indicates an anagram.
'start' means to remove the last letter (I've seen 'starts' mean this).
'to' with its last letter taken off is 't'.
't'+'rehab'='trehab'
'trehab' is an anagram of 'BATHER'.
Can you help me to learn more?
(Other definitions for bather that I've seen before include "man getting soaked?" , "one getting into the drink" , "Seaside swimmer" , "One dipping in the briny?" .)