Guy's sound external appearance (5)
I believe the answer is:
guise
'external appearance' is the definition.
(I've seen this in another clue)
'guy's sound' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'guys' is a homophone of 'GUISE'.
(Other definitions for guise that I've seen before include "Appearance, semblance" , "Shape; pretence" , "Show" , "Mask" , "Demeanour" .)