Complaints when sent mail goes astray (8)
I believe the answer is:
ailments
'complaints' is the definition.
('ailment' can be a synonym of 'complaint')
'sent mail goes astray' is the wordplay.
'goes astray' is an anagram indicator.
'sent'+'mail'='sentmail'
'sentmail' is an anagram of 'AILMENTS'.
'when' acts as a link.
(Other definitions for ailments that I've seen before include "Illnesses, complaints" , "Diseases or disorders" , "Maladies of manliest sort" , "Minor illnesses" , "Nail stem produces illnesses" .)