Olympian cheated when swimming without permit (10)
I believe the answer is:
decathlete
'olympian' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'cheated when swimming without permit' is the wordplay.
'when swimming' is an anagram indicator.
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'permit' becomes 'let' (synonyms).
'cheated' with letters rearranged gives 'decathe'.
'decathe' enclosing 'let' is 'DECATHLETE'.
Can you help me to learn more?
(Other definitions for decathlete that I've seen before include "versatile competitor" , "All-rounder" , "sportsman" .)