Complain about onset of pneumonia, or flu? (6)
I believe the answer is:
grippe
'flu?' is the definition.
(I know that flu can be written as grippe)
'complain about onset of pneumonia' is the wordplay.
'complain' becomes 'gripe' (griping is a kind of complaining).
'about' is an insertion indicator.
'onset of' says to take the initial letters.
The initial letter of 'pneumonia' is 'p'.
'gripe' placed around 'p' is 'GRIPPE'.
'or' acts as a link.
(Other definitions for grippe that I've seen before include "'flu, as before" , "infection" .)