Virile, but considerably less than the speed of sound (5)
I believe the answer is:
macho
'virile' is the definition.
(I've seen this before)
'considerably less than the speed of sound' is the wordplay.
I cannot really understand how this works, but
'speed' could be 'mach' (I have seen 'Speed of sound ' mean 'mach' so perhaps 'speed' could also mean 'mach') and 'mach' is found within the answer.
'of' could be 'o' and 'o' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'but' is the link.
Can you help me to learn more?
(Other definitions for macho that I've seen before include "Showily virile" , "Blokeish" , "Overtly masculine" , "Ultra-masculine" , "Exaggeratedly masculine" .)