Uneasiness about sea mail (7)
I believe the answer is:
malaise
'uneasiness' is the definition.
(I know that uneasiness can be written as malaise)
'about sea mail' is the wordplay.
'about' is an anagram indicator.
'sea'+'mail'='seamail'
'seamail' is an anagram of 'MALAISE'.
(Other definitions for malaise that I've seen before include "Unease; lethargy" , "Out-of-sorts feeling" , "General feeling of unease or illness" , "sickness" , "General feeling of discomfort" .)