Kind of cherry, or an extra two pounds of duck! (7)
I believe the answer is:
morello
'kind of cherry' is the definition.
'an extra two pounds of duck' is the wordplay.
'an extra' becomes 'more' (both can mean additional).
'two' says to double the letters.
'pounds' becomes 'l' (abbreviation e.g. Lsd - pounds, shillings, pence).
'of duck' becomes 'o' (I am not sure about this - if you are sure you should believe this answer much more).
'l' duplicated is 'll'.
'more'+'ll'+'o'='MORELLO'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for morello that I've seen before include "Sour red fruit" , "Type of cherry" , "Cherry's" , "Dark-red variety of cherry" , "Sour cherry tree" .)