Uncle in a smash-up! (3)
I believe the answer is:
sam
'uncle' is the definition.
(Uncle Sam is an example)
'in a smash-up' is the wordplay.
'in' indicates a hidden word.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'mas' is hidden in the letters of 'smash'.
'mas' in reverse letter order is 'SAM'.
(Other definitions for sam that I've seen before include "Boy" , "together once" , "Casablanca piano player" , "Uncle" .)