Sir Keir's companion, not good man, influencer? (7)
I believe the answer is:
charmer
'sir keir's' is the definition.
I can't judge whether this definition defines the answer.
'companion not good man influencer?' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'not good man influencer?' becomes 'armer' (I can't explain this - if you can you should give a lot more credence to this answer).
'ch'+'armer'='CHARMER'
Can you help me to learn more?
(Other definitions for charmer that I've seen before include "One working with snake" , "One having magical effect" , "One playing with snake" , "One who delights others" , "A delightful girl" .)