Where incoming mail too is no doubt welcome (7)
I believe the answer is:
outpost
'where incoming mail too is no doubt welcome' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'where incoming mail too' is the wordplay.
'where incoming' becomes 'out' (I can't explain this - if you can you should believe this answer much more).
'mail' becomes 'post' (synonyms).
'too' says to put letters next to each other.
'out'+'post'='OUTPOST'
Can you help me to learn more?
(Other definitions for outpost that I've seen before include "settlement difficult to reach?" , "Distant station or branch" , "Remote part of an empire" , "isolated settlement" , "distant location" .)