Intelligent and smooth - you don't want to be taken in by this (9)
I believe the answer is:
quicksand
'you don't want to be taken in by this' is the definition.
(it is dangerous to be stuck in quicksand)
'intelligent and smooth' is the wordplay.
'intelligent' becomes 'quick' (similar in meaning).
'and' says to put letters next to each other.
'smooth' becomes 'sand' (sanding is a kind of smoothing).
'quick'+'sand'='QUICKSAND'
(Other definitions for quicksand that I've seen before include "Treacherous situation (which sucks people and things down)" , "Treacherous wet area that swallows things up" , "Ground which sucks!" , "Treacherous terrain" , "Treacherous situation that can trap and destroy" .)