Continent, as I assume at first (4)
I believe the answer is:
asia
'continent' is the definition.
(Asia is an example)
'as i assume at first' is the wordplay.
'at first' says to take the initial letters.
The initial letter of 'assume' is 'a'.
'as'+'i'+'a'='ASIA'
(Other definitions for asia that I've seen before include "Tigers found here" , "Stretch of land" , "A is A (anag)" , "A great deal of land" , "Earth's largest continent" .)