Seize the day (5,4)
I believe the answer is:
carpe diem
'seize the day' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for carpe diem that I've seen before include "'Seize the day'" , "make the most of today" , "from Horace" , "Make the most of the present" , "Seize occasion" .)