Reckon fellow, a duke, is a leader in name only (10)
I believe the answer is:
figurehead
'a duke is a leader in name only' is the definition.
I can't judge whether this defines the answer.
'reckon fellow' is the wordplay.
'reckon' becomes 'figure' ('figure' can be a synonym of 'reckon').
'fellow' becomes 'head' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'figure'+'head'='FIGUREHEAD'
Can you help me to learn more?
(Other definitions for figurehead that I've seen before include "Front man" , "carved decoration" , "bust from the navy" , "Leader that's powerless" , "Nominal leader; carved bust at prow of ship" .)