Letters from central Cairo and Lima possibly (3,4)
I believe the answer is:
air mail
'letters' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'central cairo and lima possibly' is the wordplay.
'central' says to take the centre.
'and' means one lot of letters go next to another.
'possibly' is an anagram indicator.
The middle of 'cairo' is 'air'.
'lima' is an anagram of 'mail'.
'air'+'mail'='AIR-MAIL'
'from' is the link.
Can you help me to learn more?
(Other definitions for air mail that I've seen before include "Letter delivery system" , "It's marked 'par avion'" , "Post sent overseas" , "Letters taken by plane" , "Flying post?" .)