Sent mail about individual that's neutral (10)
I believe the answer is:
impersonal
'neutral' is the definition.
(I know that neutral can be written as impersonal)
'sent mail about individual' is the wordplay.
'sent' indicates an anagram (send can mean to excite or stimulate).
'about' means one lot of letters goes inside another.
'individual' becomes 'person' (synonyms).
'mail' is an anagram of 'imal'.
'imal' going around 'person' is 'IMPERSONAL'.
'that's' is the link.
(Other definitions for impersonal that I've seen before include "Cold" , "Aloof" , "Cool" , "Objective; disinterested" , "Ignoring one's own feelings" .)