One swimming with boy having damaged heart (6)
I believe the answer is:
bather
'one swimming' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'boy having damaged heart' is the wordplay.
'boy' becomes 'b' (abbreviation used in schools).
'having' is a charade indicator (letters next to each other).
'damaged' indicates anagramming the letters.
'heart' anagrammed gives 'ather'.
'b'+'ather'='BATHER'
'with' is the link.
Can you help me to learn more?
(Other definitions for bather that I've seen before include "Seaside swimmer" , "one getting into the drink" , "man getting soaked?" , "One dipping in the briny?" .)