Wagging tongues in your heartless youth (9)
I believe the answer is:
youngster
'youth' is the definition.
(I've seen this before)
'wagging tongues in your heartless' is the wordplay.
'wagging' indicates an anagram (I've seen this in other clues).
'in' is an insertion indicator.
'heartless' says to hollow out the word (remove centre letters).
'your' with its middle taken out is 'yr'.
'tongues' is an anagram of 'oungste'.
'oungste' inserted within 'yr' is 'YOUNGSTER'.
(Other definitions for youngster that I've seen before include "Stripling" , "Child, minor" , "Rug rat in New York" , "Person of tender years" , "one of tender years" .)