Unpolluted Incan lakesides (5)
I believe the answer is:
clean
'unpolluted' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for clean that I've seen before include "Free from dirt; well defined" , "Honest" , "Quite" , "attentive to personal hygiene" , "thoroughly" .)