Measure area with one large duck (7)
I believe the answer is:
pintail
'duck' is the definition.
(I know that pintail is a type of duck)
'measure area with one large' is the wordplay.
'measure' becomes 'pint' (pint is a kind of measure).
'area' becomes 'a' (maths abbreviation).
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'large' becomes 'L' (eg in clothes sizes).
'pint'+'a'+'i'+'l'='PINTAIL'
(Other definitions for pintail that I've seen before include "Species of duck" , "Breed of duck" .)