Area covered by most suitable animal (5)
I believe the answer is:
beast
'animal' is the definition.
(I know that animal can be written as beast)
'area covered by most suitable' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'covered by' indicates putting letters inside.
'most suitable' becomes 'best' (I've seen this in another clue).
'a' placed into 'best' is 'BEAST'.
(Other definitions for beast that I've seen before include "Large animal -- objectionable person" , "Pig" , "Wild animal" , "Bully" , "Savage" .)