Thick dust leads to influenza patients' suffering (6)
I believe the answer is:
stupid
'thick' is the definition.
(similar in meaning)
'dust leads to influenza patients' suffering' is the wordplay.
'leads to' says to take the initial letters.
'suffering' is an anagram indicator.
The first letters of 'influenza patients' is 'ip'.
'dust'+'ip'='dustip'
'dustip' is an anagram of 'STUPID'.
(Other definitions for stupid that I've seen before include "Not sensible or intelligent" , "dope" , "Idiotic" , "Lacking in common sense" , "Obtuse" .)