To do so weakens one's spirit (6)
I believe the answer is:
dilute
'spirit' is the definition.
The definition and answer 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?
'to do so weakens one's' is the wordplay.
I cannot quite understand how this works, but
'do' could be 'ut' (synonyms) and 'ut' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for dilute that I've seen before include "Thin out, make less strong" , "having less concentration" , "Weaken by adding water" , "Make (a liquid) thinner" , "not neat" .)