River claiming last of Olympian youth (5)
I believe the answer is:
teens
'youth' is the definition.
(I've seen this in another clue)
'river claiming last of olympian' is the wordplay.
'river' becomes 'tees' (River Tees).
'claiming' means one lot of letters goes inside another.
'last of' says to take the final letters.
The last letter of 'olympian' is 'n'.
'tees' going around 'n' is 'TEENS'.
(Other definitions for teens that I've seen before include "Years 13-19" , "children" , "Adolescence" , "13-19 age group" , "seven-year period" .)