Removing the extra mailbox arrangement is a logical starting point (5)
I believe the answer is:
axiom
'logical starting point' is the definition.
(I've seen this in another clue)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for axiom that I've seen before include "Generally accepted proposition" , "Accepted statement taken as self-evidently true" , "Accepted general truth or principle" , "fundamental principle" , "Plainly true proposition" .)