Not confident, making changes by force (9)
I believe the answer is:
faltering
'not confident' is the definition.
I can't judge whether this definition defines the answer.
'making changes by force' is the wordplay.
'making changes' becomes 'altering' (I've seen this before).
'by' means one lot of letters go next to another.
'force' becomes 'f' (abbreviation in physics e.g. f=ma).
'altering' after 'f' is 'FALTERING'.
Can you help me to learn more?
(Another definition for faltering that I've seen is " Uncertain".)