Almost caught flu (6)
I believe the answer is:
grippe
'flu' is the definition.
(I know that flu can be written as grippe)
'almost caught' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'caught' becomes 'gripped' (grip can mean to catch).
'gripped' with its last letter removed is 'GRIPPE'.
(Other definitions for grippe that I've seen before include "'flu, as before" , "infection" .)