Fail to finish off note about your requirement when getting a flat (5,4)
I believe the answer is:
spare tyre
'requirement when getting a flat' is the definition.
(used when you have a flat tyre)
'fail to finish off note about your' is the wordplay.
'fail to finish off' becomes 'spare' (as in sparing someone's life).
'note' becomes 'te' (musical note).
'about' indicates putting letters inside.
'your' becomes 'yr'.
'te' going around 'yr' is 'tyre'.
'spare'+'tyre'='SPARE TYRE'
(Other definitions for spare tyre that I've seen before include "Roll of fat around the tummy" , "Middle-age spread" , "Part of corporation" , "large corporation" , "Form of extra weight" .)