Granny, initially out and back in practically no time! (10)
I believe the answer is:
nanosecond
'practically no time' is the definition.
(a very small amount of time)
'granny initially out and back' is the wordplay.
'granny' becomes 'nan'.
'initially' says to take the initial letters.
'and' says to put letters next to each other.
'back' becomes 'second' (both can mean to support an idea or proposal).
The first letter of 'out' is 'o'.
'nan'+'o'+'second'='NANOSECOND'
'in' acts as a link.
(Other definitions for nanosecond that I've seen before include "Only a little flash" , "next to no time!" , "Tiny duration" , "instant?" , "A very short interval of time" .)