Underscore young people, being eager, sent flying (9)
I believe the answer is:
teenagers
'underscore young people' is the definition.
(teenagers are under 20 - 'score' can mean 20)
'eager sent flying' is the wordplay.
'flying' indicates an anagram (letters fly around into new positions).
'eager'+'sent'='eagersent'
'eagersent' is an anagram of 'TEENAGERS'.
'being' is the link.
(Other definitions for teenagers that I've seen before include "They're aged between 13 and 19" , "They're between the onset of puberty and maturity, generally" , "Adolescents" , "Youngsters" , "Young people" .)