Sea mail going astray causes uneasiness (7)
I believe the answer is:
malaise
'uneasiness' is the definition.
(I know that uneasiness can be written as malaise)
'sea mail going astray' is the wordplay.
'going astray' indicates anagramming the letters.
'sea'+'mail'='seamail'
'seamail' is an anagram of 'MALAISE'.
'causes' acts as a link.
(Other definitions for malaise that I've seen before include "Feeling of illness or mental uneasiness" , "Out-of-sorts feeling" , "General feeling of unease or illness" , "flu perhaps" , "depression" .)