I will block return of unimpressive spam? (1-4)
I believe the answer is:
'spam?' is the definition.
(spam is a type of e-mail)
'i will block return of unimpressive' is the wordplay.
'will block' is an insertion indicator.
'return of' says the letters should be written in reverse.
'unimpressive' becomes 'lame'.
'lame' in reverse letter order is 'emal'.
'i' put inside 'emal' is 'E-MAIL'.
(Other definitions for e-mail that I've seen before include "Online message" , "Fast communication" , "Digital message" , "Modern method of communication via Internet" , "Send a message" .)