Miles covered by darn duck (4)
I believe the answer is:
smew
'duck' is the definition.
(smew is a kind of duck)
'miles covered by darn' is the wordplay.
'miles' becomes 'm' (common abbreviation).
'covered by' means one lot of letters goes inside another.
'darn' becomes 'sew' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'm' placed inside 'sew' is 'SMEW'.
Can you help me to learn more?
(Other definitions for smew that I've seen before include "Sea duck" , "Small merganser" , "Bird" , "Diver" , "Species of merganser" .)