Former head's term of endearment (3,4)
I believe the answer is:
old bean
'term of endearment' is the definition.
I know nothing about this answer so I can't tell whether this works.
'former head's' is the wordplay.
'former' becomes 'old' (both can mean previous).
'head' becomes 'bean' (bean is a kind of head).
'old'+'bean'='OLD BEAN'
Can you help me to learn more?
(Other definitions for old bean that I've seen before include "My dear chap!" , "dear fellow" , "Term of affection for a man" , "Dated familiar term of address to a man" , "Familiar address to chap" .)