What young often do in earnest — learn (6)
I believe the answer is:
nestle
'what young often do' is the definition.
(eg a baby nestles in its mother's arms)
'in earnest learn' is the wordplay.
'in' indicates a hidden word.
'NESTLE' is hidden within 'earnest learn'.
(Other definitions for nestle that I've seen before include "Settle snugly (within)" , "Snuggle up, settle in comfortably" , "Lie safely" , "Swiss food and drink company" , "Press close and snugly" .)