Not without a time limit! (5)
I believe the answer is:
never
'not' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for never that I've seen before include "Not in a month of Sundays" , "At no time in the past or future" , "' - - Say - - Again': Bond film" , "I refuse" , "Surely not" .)