Mammal frantic in a small stream with reduced oxygen (9)
I believe the answer is:
armadillo
'mammal' is the definition.
(I've seen this before)
'frantic in a small stream with reduced oxygen' is the wordplay.
'frantic' becomes 'mad' ('mad' can be a synonym of 'frantic').
'in' is an insertion indicator.
'small stream' becomes 'rill' (I've seen this before).
'with reduced oxygen' becomes 'o' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'rill'='arill'
'mad' going into 'arill' is 'armadill'.
'armadill'+'o'='ARMADILLO'
(Other definitions for armadillo that I've seen before include "protected species" , "Bony-plated mammal" , "Scaly mammal" , "Nocturnal creature" , "American insect-eater" .)