Unwanted mail from American uncle, penning page (4)
I believe the answer is:
spam
'unwanted mail' is the definition.
(junk email)
'american uncle penning page' is the wordplay.
'american uncle' becomes 'Sam' (Uncle Sam is a nickname for the USA).
'penning' indicates putting letters inside.
'page' becomes 'p'.
'sam' going around 'p' is 'SPAM'.
'from' is the link.
(Other definitions for spam that I've seen before include "Junk emails" , "Tinned processed meat" , "Unwelcome approach" , "net on junk? On the contrary" , "Unwanted communication" .)