Inflexible, adult bones (6)
I believe the answer is:
sterna
'bones' is the definition.
(more than one sternum)
'inflexible adult' is the wordplay.
'inflexible' becomes 'stern' (similar in meaning).
'adult' becomes 'a' (abbreviation).
'stern'+'a'='STERNA'
(Other definitions for sterna that I've seen before include "Formal backbones, Latin plural" , "Breastbones" , "Bones in front" .)