A bather may do it if he can't stand swimming (5,5)
I believe the answer is:
tread water
'stand swimming' is the definition.
'tread water' can be an answer for 'stand' (thesaurus). I'm not certain of the 'swimming' bit.
'a bather may do it if he can't' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
'he' could be 'wat' and 'wat' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tread water that I've seen before include "Mainly make no progress" , "Hold back from making progress" , "Stay afloat" , "Make little or no progress" , "just survive" .)