Offer to include upset batman down the order (9)
I believe the answer is:
tailender
'batman down the order' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'offer to include upset' is the wordplay.
'offer' becomes 'tender' (tender can mean to offer something).
'to include' means one lot of letters goes inside another.
'upset' becomes 'ail' (I've seen this before).
'tender' enclosing 'ail' is 'TAILENDER'.
Can you help me to learn more?
(Other definitions for tailender that I've seen before include "Rabbit" , "one of the last to come in" .)