Bit of a charmer, old Macmillan was (6)
I believe the answer is:
harold
'macmillan was' is the definition.
I can't judge whether this defines the answer.
'bit of a charmer old' is the wordplay.
'bit of a charmer' becomes 'har' (I can't explain this - if you can you should give a lot more credence to this answer).
'har'+'old'='HAROLD'
Can you help me to learn more?
(Other definitions for harold that I've seen before include "King of England, January-October, 1066" , "King of England 1037-40" , "Maybe Steptoe" , "Last Anglo-Saxon king of England" , "King killed at Hastings" .)