Marsupial known at first over a large area (5)
I believe the answer is:
koala
'marsupial' is the definition.
(koala is a kind of marsupial)
'known at first over a large area' is the wordplay.
'at first' says to take the initial letters.
'over' becomes 'o' (cricket abbreviation).
'large' becomes 'l'.
'area' becomes 'a' (maths abbreviation).
The initial letter of 'known' is 'k'.
'k'+'o'+'a'+'l'+'a'='KOALA'
(Other definitions for koala that I've seen before include "Tree-hugger" , "Cuddly Australian marsupial" , "Australian marsupial which lives on eucalyptus leaves" , "small bear" , "Australian creature" .)