Flatten a large adult bear? Apparently not! (5)
I believe the answer is:
koala
'bear? apparently not' is the definition.
I can't tell whether this definition defines the answer.
'flatten a large adult' is the wordplay.
'flatten' becomes 'koa' (I can't explain this - if you can you should give a lot more credence to this answer).
'a large' becomes 'L' (eg in clothes sizes).
'adult' becomes 'a' (abbreviation).
'koa'+'l'+'a'='KOALA'
Can you help me to learn more?
(Other definitions for koala that I've seen before include "Marsupial "bear"" , "leaf-eater" , "One's usually up a tree" , "Bear-like Australian marsupial" , "Australian creature" .)