Chap in a smash up (3)
I believe the answer is:
sam
'chap' is the definition.
(man's name)
'in a smash up' is the wordplay.
'in' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'mas' can be found hidden inside 'smash'.
'mas' backwards is 'SAM'.
(Other definitions for sam that I've seen before include "together once" , "Casablanca piano player" , "Uncle" , "Boy" .)