Saw rare Australian mammal with one but managed to keep the head down over invoice (8)
I believe the answer is:
duckbill
'saw rare australian mammal with one but managed to keep' is the definition.
I can't judge whether this definition defines the answer.
'head down over invoice' is the wordplay.
'head down' becomes 'duck' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'invoice' becomes 'bill' (synonyms).
'duck'+'bill'='DUCKBILL'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for duckbill that I've seen before include "Platypus" , "uniquely antipodean creature" .)