Complain about onset of pig flu (6)
I believe the answer is:
grippe
'flu' is the definition.
(I've seen this before)
'complain about onset of pig' is the wordplay.
'complain' becomes 'gripe' (griping is a kind of complaining**).
'about' means one lot of letters goes inside another.
'onset of' indicates taking the first letters.
The initial letter of 'pig' is 'p'.
'gripe' going around 'p' is 'GRIPPE'.
(Other definitions for grippe that I've seen before include "infection" , "'flu, as before" .)