Depressing affair with the French flu spreading (7)
I believe the answer is:
doleful
'depressing' is the definition.
(thesaurus)
'affair with the french flu spreading' is the wordplay.
'affair' becomes 'do' (do is a kind of affair).
'with' is an insertion indicator.
'the french' becomes 'le' ('the' in French).
'spreading' is an anagram indicator.
'flu' is an anagram of 'ful'.
'le' placed into 'ful' is 'leful'.
'do'+'leful'='DOLEFUL'
(Other definitions for doleful that I've seen before include "Melancholic" , "Miserable" , "Lugubrious" , "Dreary" , "not happy?" .)