See us with uncle, as a rule (6)
I believe the answer is:
custom
'a rule' is the definition.
(thesaurus)
'see us with uncle' is the wordplay.
'see' becomes 'C' (the word for the letter, according to Chambers).
'with' says to put letters next to each other.
'uncle' becomes 'tom' (I can't explain this - if you can you should give a lot more credence to this answer).
'c'+'us'+'tom'='CUSTOM'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for custom that I've seen before include "Regular process" , "Usual way of doing something" , "Usual practice" , "Habitual action" , "Accepted or habitual practice" .)