One in the habit of taking stock? (7)
I believe the answer is:
rustler
This might be a double definition.
'one' is the first definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'habit of taking stock?' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'in the' is the link.
(Other definitions for rustler that I've seen before include "Stealer of livestock" , "Cattle thief" , "Cattle-robber" , "one whose stock-taking is dishonest" , "Cattle-thief" .)