One having systematic way to do things, hunting animal (6)
I believe the answer is:
mouser
'one having systematic way to do things hunting' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'animal' is the wordplay.
I cannot really see how this works, but
'animal' could be 'mouse' (mouse is a kind of animal) and 'mouse' is found in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for mouser that I've seen before include "One getting rid of vermin" , "Cat with certain skills" , "Useful mog" , "Preying cat" , "Cat proficient at catching small rodents" .)