Turning up, snatch block (3)
I believe the answer is:
ban
'block' is the definition.
(I've seen this in another clue)
'turning up snatch' is the wordplay.
'turning up' says the letters should be written in reverse.
'snatch' becomes 'nab' (I've seen this in another clue).
'nab' backwards is 'BAN'.
(Other definitions for ban that I've seen before include "the calling of young vassals" , "Veto" , "outlaw" , "Stop" , "Censor or proscribe" .)