Fast speed over endlessly polluted air in a show of swagger (8)
I believe the answer is:
'a show of swagger' is the definition.
(stereotypically manly behaviour)
'fast speed over endlessly polluted air' is the wordplay.
'fast speed' becomes 'Mach I' (as in Mach 1).
'over' says to put letters next to each other.
'endlessly' means to remove the last letter.
'polluted air' becomes 'smog' (smog is fog caused by pollution).
'smog' with its final letter taken away is 'smo'.
'in' acts as a link.
(Other definitions for machismo that I've seen before include "Exaggerated sense of manliness" , "A show of virility or manliness" , "The cult of masculine pride" , "show of masculinity" , "Hi, commas, strangely, might have exaggerated masculinity" .)