Though not altruistic, Les backs a swimmer (7)
I believe the answer is:
selfish
'though not altruistic' is the definition.
I can't judge whether this definition defines the answer.
'les backs a swimmer' is the wordplay.
'backs' shows that the letters should be reversed in order.
'a swimmer' becomes 'fish' (fish swim).
'les' in reverse letter order is 'sel'.
'sel'+'fish'='SELFISH'
Can you help me to learn more?
(Other definitions for selfish that I've seen before include "Egocentric - mean" , "Egotistical" , "Concerned only with oneself" , "Mercenary" , "without consideration" .)