At the speed of sound, two seconds of manliness (8)
I believe the answer is:
machismo
'manliness' is the definition.
(I've seen this in another clue)
'at the speed of sound two seconds' is the wordplay.
'two seconds' can mean two replacements for 'second'.
'at the speed of sound' becomes 'Mach I' (Mach 1 is the speed of sound).
'second' becomes 's' (common abbreviation).
'second' becomes 'mo' (both are short periods of time).
'machi'+'s'+'mo'='MACHISMO'
'of' acts as a link.
(Other definitions for machismo that I've seen before include "large injection of testosterone!" , "form of pride" , "A show of virility or manliness" , "Strong masculine pride" , "Expression of male pride" .)