Formally argued, indeed, about a flying mammal (7)
I believe the answer is:
debated
'formally argued' is the definition.
'debated' can be an answer for 'argued' ('debate' can be a synonym of 'argue'). I am not sure about the 'formally' bit.
'indeed about a flying mammal' is the wordplay.
'indeed' becomes 'deed' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'a flying mammal' becomes 'bat' (I've seen this in another clue).
'deed' enclosing 'bat' is 'DEBATED'.
Can you help me to learn more?
(Other definitions for debated that I've seen before include "Shared opinions" , "Discussed formally" , "Considered" , "Argued" , "Contemplated" .)