Smash mad character supporting socialist's leader (7)
I believe the answer is:
shatter
'smash' is the definition.
(I've seen this in another clue)
'mad character supporting socialist's leader' is the wordplay.
'mad character' becomes 'hatter' (Alice In Wonderland).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'leader' suggests taking the first letters.
The first letter of 'socialists' is 's'.
'hatter' put after 's' is 'SHATTER'.
(Other definitions for shatter that I've seen before include "Break into many pieces" , "Crack" , "Break the minister into little pieces" , "Support for broken arm" , "thoroughly upset" .)