Mail was reduced regularly, primarily from late ’40s? (4-3)
I believe the answer is:
post-war
'late '40s?' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'mail was reduced regularly primarily' is the wordplay.
'mail' becomes 'post' (synonyms).
'reduced' means to remove the last letter.
'primarily' suggests taking the first letters.
The initial letter of 'regularly' is 'r'.
'was' with its last letter taken off is 'wa'.
'post'+'wa'+'r'='POST-WAR'
'from' is the link.
(Other definitions for post-war that I've seen before include "After 1918 or 1945?" , "Since 1945" , "Eg, after 1945" , "Occurring after an armed conflict" , "After, say, 1945" .)