Criminal activity served up in casinos, rampant (5)
I believe the answer is:
arson
'criminal activity' is the definition.
(I've seen this before)
'up in casinos rampant' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' says the answer is hidden in the clue.
'nosra' can be found hidden inside 'casinos rampant'.
'nosra' in reverse letter order is 'ARSON'.
'served' acts as a link.
(Other definitions for arson that I've seen before include "Iniquitous behaviour" , "Fire-lighting crime" , "burning problem" , "criminal fire-raising" , "Incendiary act" .)