Summons? Nothing to us, being hardened (7)
I believe the answer is:
callous
'being hardened' is the definition.
('callous' can be a synonym of 'hardened')
'summons? nothing to us' is the wordplay.
'summons?' becomes 'call' (summonsing is a kind of calling).
'nothing' becomes 'o' (looks like zero - 0).
'call'+'o'+'us'='CALLOUS'
(Other definitions for callous that I've seen before include "Cruelly unfeeling" , "Hard and unfeeling" , "Emotionally hardened" , "Cruel and unfeeling" , "Insensitive, cruel" .)