Callous — as Humpty Dumpty might have been? (4-6)
I believe the answer is:
hard-boiled
'callous' is the definition.
(synonyms)
'humpty dumpty might have been?' is the wordplay.
I cannot quite understand how this works, but
'been?' could be 'led' (leading is a kind of being) and 'led' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'as' is the link.
(Other definitions for hard-boiled that I've seen before include "Unsentimental" , "Tough (as an egg can be?)" , "denying entry to soldiers?" , "way to serve an egg?" , "Cynical" .)