Sweetener involved in car chase (10)
I believe the answer is:
saccharine
'sweetener' is the definition.
(I've seen this before)
'involved in car chase' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'in'+'car'+'chase'='incarchase'
'incarchase' anagrammed gives 'SACCHARINE'.
(Other definitions for saccharine that I've seen before include "Sickly" , "Sugary" , "China cares (anag.)" , "Excessively sweet" , "Excessively sentimental" .)