Available for one worried about onset of flu (2,5)
I believe the answer is:
on offer
'available' is the definition.
(I've seen this before)
'for one worried about onset of flu' is the wordplay.
'worried' is an anagram indicator.
'about' indicates putting letters inside.
'onset of' suggests taking the first letters.
The first letter of 'flu' is 'f'.
'for'+'one'='forone'
'forone' is an anagram of 'onofer'.
'onofer' going around 'f' is 'ON OFFER'.
(Other definitions for on offer that I've seen before include "is made cheaper?" , "For sale at a certain price" , "For sale at specified price" , "available for less" , "At special price" .)