Plain awful to be full of flu essentially (4)
I believe the answer is:
bald
'plain' is the definition.
(both can mean unadorned)
'awful to be full of flu essentially' is the wordplay.
'awful' becomes 'bad' (similar in meaning).
'to be full of' is an insertion indicator.
'essentially' means to look at the middle letters.
The centre of 'flu' is 'l'.
'bad' enclosing 'l' is 'BALD'.
(Other definitions for bald that I've seen before include "Exposed" , "With no effort to conceal" , "Plain; hairless" , "Without hair" , "Lacking hair on the scalp" .)