Young people last to drown in river (5)
I believe the answer is:
teens
'young people' is the definition.
(I've seen this in another clue)
'last to drown in river' is the wordplay.
'last to' says to take the final letters.
'in' is an insertion indicator.
'river' becomes 'tees' (River Tees).
The last letter of 'drown' is 'n'.
'n' going inside 'tees' is 'TEENS'.
(Other definitions for teens that I've seen before include "13-19 age group" , "Between 12 and 20" , "seven-year period" , "Years 13-19" , "children" .)