Name a small number after one in ultrashort time (10)
I believe the answer is:
nanosecond
'ultrashort time' is the definition.
(very short period of time)
'name a small number after one' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'small number' becomes 'no' (abbreviation for 'number').
'after one' becomes 'second' (in second place).
'n'+'a'+'no'+'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!" , "A very short interval of time" , "trice" , "very brief time" .)