Initially shocking commotion, with chap circling extinct mammal (8)
I believe the answer is:
mastodon
'extinct mammal' is the definition.
(I've seen this in another clue)
'initially shocking commotion with chap' is the wordplay.
'initially shocking commotion' becomes 'mason' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters goes inside another.
'chap' becomes 'Tod'.
'mason' placed around 'tod' is 'MASTODON'.
'circling' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for mastodon that I've seen before include "Extinct mammal like an elephant" , "Huge animal" , "giant from the past" , "Big beast" , "Extinct beast" .)