Handy flu shot requires application (6)
I believe the answer is:
useful
'handy' is the definition.
(I've seen this before)
'flu shot requires application' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'flu' is 'ful' which is within the answer.
'application' could be 'use' (I've seen this in another clue) and 'use' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for useful that I've seen before include "Effective" , "Handy, practical" , "Advantageous - efficacious" , "Practicable" , "Utilitarian" .)