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