Use of it may weaken the spirit (6)
I believe the answer is:
siphon
This might be a double definition.
'use' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'it may weaken the spirit' is the second definition.
The definition suggests a singular noun which matches the answer.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for siphon that I've seen before include "Bottle from which a liquid is forced by pressure of gas" , "Tube drawing off liquid" , "Tube used to draw a liquid" , "Soda dispenser" , "Kind of bottle for squirting, say soda" .)