Anxiety caused by sea mail going astray (7)
I believe the answer is:
malaise
'anxiety' is the definition.
(thesaurus)
'sea mail going astray' is the wordplay.
'going astray' indicates an anagram.
'sea'+'mail'='seamail'
'seamail' anagrammed gives 'MALAISE'.
'caused by' acts as a link.
(Other definitions for malaise that I've seen before include "Out-of-sorts feeling" , "General feeling of unease or illness" , "Uneasy feeling" , "General feeling of discomfort" , "Unease; lethargy" .)