Eat about ten, finish pie, leave (5)
I believe the answer is:
exeat
'leave' is the definition.
(I've seen this before)
'eat about ten finish pie' is the wordplay.
'about' indicates putting letters inside.
'ten' becomes 'x' (Roman numeral).
'finish' indicates one should take the final letters.
The last letter of 'pie' is 'e'.
'x'+'e'='xe'
'eat' placed around 'xe' is 'EXEAT'.
(Other definitions for exeat that I've seen before include "Leave of absence" , "Permission to be absent from college/school" , "Permission to go" , "... [23A=PASS]" , "permission to leave" .)