Individual friend, in my estimation (10)
I believe the answer is:
personally
'my estimation' is the definition.
I can't judge whether this defines the answer.
'individual friend' is the wordplay.
'individual' becomes 'person' (an individual person).
'friend' becomes 'ally' (an ally is a friend or supporter).
'person'+'ally'='PERSONALLY'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for personally that I've seen before include "in my opinion" , "Speaking for myself" , "in my book" , "I think" , "as far as one is concerned" .)