Oxygen's needed by fool grabbing tail of savage cat (6)
I believe the answer is:
ocelot
'cat' is the definition.
(ocelot is a kind of cat)
'oxygen's needed by fool grabbing tail of savage' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'needed by' means one lot of letters go next to another.
'fool' becomes 'clot' (I've seen this before**).
'grabbing' is an insertion indicator.
'tail of' says to take the final letters.
The final letter of 'savage' is 'e'.
'clot' enclosing 'e' is 'celot'.
'o'+'celot'='OCELOT'
(Other definitions for ocelot that I've seen before include "Nocturnal wild cat of Central and South America" , "Small leopardlike cat" , "Striped and spotted wildcat" , "Wildcat -- loo etc (anag)" , "Yellow-black striped and spotted cat" .)