Moment; credit (4)
I believe the answer is:
tick
This clue is a double definition.
'moment' is the first definition.
(both can mean a very short amount of time)
'credit' is the second definition.
(on tick can mean purchased on credit)
(Other definitions for tick that I've seen before include "Credit (sl.)" , "Parasite - mark of approval" , "Credit; mark as correct" , "Second" , "Mark indicating correctness" .)