Back in half a second; a very brief space of time (5)
I believe the answer is:
flash
'time' is the definition.
(I know that flash is a type of bit)
'back in half a second' is the wordplay.
'back' shows that the letters should be reversed in order.
'in' indicates putting letters inside.
'a second' becomes 's' (common abbreviation).
'half' placed around 's' is 'hsalf'.
'hsalf' back-to-front is 'FLASH'.
'a very brief space of' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for flash that I've seen before include "Streak" , "Instant" , "beam" , "type of photography" , "sort of flooding" .)