Hardened to a certain amount of pain and a rude shock! (6)
I believe the answer is:
inured
'hardened' is the definition.
('inure' can be a synonym of 'harden')
'a certain amount of pain and a rude shock' is the wordplay.
I cannot really see how this works, but
'amount' could be 'in' (in is a kind of amount) and 'in' is found in the answer.
an anagram of 'rude' is 'ured' which is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for inured that I've seen before include "Hardened as by experience" , "Hardened against, toughened" , "Hardened or calloused, being ruined" , "Hardened, perhaps ruined" , "Accustomed to hardship" .)