Bond is old hat, possibly (4)
I believe the answer is:
oath
'bond' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'old hat possibly' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'possibly' indicates an anagram.
'hat' with letters rearranged gives 'ath'.
'o'+'ath'='OATH'
'is' is the link.
Can you help me to learn more?
(Other definitions for oath that I've seen before include "Imprecation" , "Curse - sworn statement" , "Swearword or solemn promise" , "It could be ruddy when minced" , "Swearword, expletive" .)