Giving in to flu bout, occasionally nasty (9)
I believe the answer is:
bountiful
'giving' is the definition.
('bountiful' can be a synonym of 'giving')
'in to flu bout occasionally nasty' is the wordplay.
'occasionally' indicates alternate letters.
'nasty' indicates an anagram.
The alternate letters of 'bout' are 'bu'.
'in'+'to'+'flu'+'bu'='intoflubu'
'intoflubu' is an anagram of 'BOUNTIFUL'.
(Other definitions for bountiful that I've seen before include "where Carrie Watts grew up" , "Ample or generous (Lady!)" , "Giving generously" , "Generous in giving -- abundant" , "flush with bonus for joining up?" .)