Become hard and bony with age (6)
I believe the answer is:
ossify
'become hard and bony with age' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ossify that I've seen before include "become inflexible" , "Solidify; turn into bone" , "stop development" , "Change into bone, become rigid" , "Turn to bone with age" .)