Friendly letters in email: A, B, C (8)
I believe the answer is:
amicable
'friendly' is the definition.
(associated in meaning)
'letters in email a b c' is the wordplay.
'letters in' indicates an anagram.
'email'+'a'+'b'+'c'='emailabc'
'emailabc' with letters rearranged gives 'AMICABLE'.
(Other definitions for amicable that I've seen before include "Friendly, nonthreatening" , "Male cab I change to, just to be friendly" , "Friendly, unthreatening" , "on good terms" , "friendliness" .)