Coming up, forbid arrest (3)
I believe the answer is:
nab
'arrest' is the definition.
(I know that arrest can be written as nab)
'coming up forbid' is the wordplay.
'coming up' shows that the letters should be reversed in order (in a down clue, letters go up).
'forbid' becomes 'ban' (banning is a kind of forbiding).
'ban' in reverse letter order is 'NAB'.
(Other definitions for nab that I've seen before include "Catch (thief) (colloq.)" , "Catch; grab suddenly" , "Seize quickly" , "nail" , "Cop" .)