His E-mail, it seems, has a letter missing (4)
I believe the answer is:
emil
'his e-mail it seems' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'a letter missing' is the wordplay.
'a letter' becomes 'em' (I've seen this in other clues).
'missing' becomes 'il' (I can't justify this - if you can you should believe this answer much more).
'em'+'il'='EMIL'
'has' is the link.
Can you help me to learn more?
(Other definitions for emil that I've seen before include "- - Zatopek, Olympic runner" , "French boy's name" , "German boy" .)