Instinct used in casinos effectively (4)
I believe the answer is:
nose
'instinct' is the definition.
(a nose for something is a good instinct for it)
'in casinos effectively' is the wordplay.
'in' indicates a hidden word.
'NOSE' can be found hidden inside 'casinos effectively'.
'used' acts as a link.
(Other definitions for nose that I've seen before include "Hooter" , "Don't cut this off to spite your face" , "Proboscis" , "Facial part" , "Front of plane" .)