A herb finally mailed out (6)
I believe the answer is:
absent
'out' is the definition.
(similar in meaning)
'a herb finally mailed' is the wordplay.
'finally' says to take the final letters.
'mailed' becomes 'sent' ('send' can be a synonym of 'mail').
The final letter of 'herb' is 'b'.
'a'+'b'+'sent'='ABSENT'
(Other definitions for absent that I've seen before include "lack of attention" , "Inattentive" , "Not where one should be" , "Away" , "Not present or focused" .)