Temperature has seen off youngsters (5)
I believe the answer is:
teens
'youngsters' is the definition.
(I've seen this in another clue)
'temperature has seen off' is the wordplay.
'temperature' becomes 't' (abbreviation).
'has' means one lot of letters go next to another.
'off' indicates anagramming the letters.
'seen' with letters rearranged gives 'eens'.
't'+'eens'='TEENS'
(Other definitions for teens that I've seen before include "Between 12 and 20" , "Adolescence" , "The years 13 to 19" , "13-19 age group" , "Adolescent years" .)