When reckoning by English Duke is fictitious (7)
I believe the answer is:
assumed
'fictitious' is the definition.
(synonyms)
'when reckoning by english duke' is the wordplay.
'when' becomes 'as'.
'reckoning by' becomes 'sum' (I am not sure about this - if you are sure you should believe this answer much more).
'english' becomes 'e' (abbreviation).
'duke' becomes 'd'.
'as'+'sum'+'e'+'d'='ASSUMED'
'is' is the link.
Can you help me to learn more?
(Other definitions for assumed that I've seen before include "Affected; supposed" , "Fancied" , "Taken for granted - feigned" , "False" , "Took for granted" .)