Big beasts somehow adapting as necessary originally (5,6)
I believe the answer is:
giant pandas
'big beasts' is the definition.
(giant pandas are large animals)
'somehow adapting as necessary originally' is the wordplay.
'somehow' indicates an anagram.
'originally' says to take the initial letters.
The first letter of 'necessary' is 'n'.
'adapting'+'as'+'n'='adaptingasn'
'adaptingasn' is an anagram of 'GIANT PANDAS'.
(Other definitions for giant pandas that I've seen before include "Mammals" , "Animals" .)