Former head, a classy chap (3,4)
I believe the answer is:
old bean
'a classy chap' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'former head' is the wordplay.
'former' becomes 'old' (I've seen this before).
'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 "Good friend" , "My dear chap!" , "Dated familiar term of address to a man" , "Familiar address to chap" , "Familiar way to address" .)