Judah’s son up and back in extremely short period (10)
I believe the answer is:
nanosecond
'extremely short period' is the definition.
The answer and definition can be both related to time as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'judah's son up and back' is the wordplay.
'judah's son' becomes 'onan' (biblical figure).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'and' is a charade indicator (letters next to each other).
'back' becomes 'second' (both can mean to support an idea or proposal).
'onan' backwards is 'nano'.
'nano'+'second'='NANOSECOND'
'in' is the link.
Can you help me to learn more?
(Other definitions for nanosecond that I've seen before include "Only a little flash" , "trice" , "instant?" , "very brief time" , "Tiny duration" .)