Medicare provision originally organised — it's now or never! (5,4)
I believe the answer is:
carpe diem
'it's now or never' is the definition.
('carpe diem' means 'seize the day' in Latin)
'medicare provision originally organised' is the wordplay.
'originally' indicates taking the first letters.
'organised' indicates anagramming the letters.
The first letter of 'provision' is 'p'.
'medicare'+'p'='medicarep'
'medicarep' with letters rearranged gives 'CARPE DIEM'.
(Other definitions for carpe diem that I've seen before include "from Horace" , "Make the most of the present" , "Gather ye rosebuds" , "'Seize the day'" , "Enjoy the moment! (Latin)" .)