Weaken the spirit (6)
I believe the answer is:
dilute
This clue could be a double definition.
'weaken' is the first definition.
(I know that dilute is a more specific form of the action the weaken)
'spirit' is the second definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Perhaps they are linked in a way I don't understand?
'the' acts as a link.
Can you help me to learn more?
(Other definitions for dilute that I've seen before include "having less concentration" , "Adulterate" , "Reduce concentration of a liquid" , "Add liquid to lessen strength" , "Add water to another liquid" .)