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' says the answer is hidden in the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'mas' is hidden within 'smash'.
'mas' written backwards gives 'SAM'.
(Other definitions for sam that I've seen before include "Boy" , "together once" , "Casablanca piano player" , "Uncle" .)