Bones are hard with adults (6)
I believe the answer is:
sterna
'bones' is the definition.
(sternum is a kind of bone)
'hard with adults' is the wordplay.
'hard' becomes 'stern' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'adults' becomes 'a' (this could be a standard abbreviation of which I'm unaware).
'stern'+'a'='STERNA'
'are' acts as a link.
Can you help me to learn more?
(Other definitions for sterna that I've seen before include "Breastbones" , "Bones in front" , "Formal backbones, Latin plural" .)