Very small interval — much less than an hour (6)
I believe the answer is:
minute
This clue is a double definition.
'very small' is the first definition.
(minute can mean tiny)
'interval much less than an hour' is the second definition.
(a minute is less than an hour)
(Other definitions for minute that I've seen before include "Exact" , "Tiny; period of time" , "Precise" , "Small, tiny" , "Short period of time" .)