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' means one lot of letters go next to another.
'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" .)