Crooks assembled this eve (7)
I believe the answer is:
thieves
'crooks' is the definition.
(thief is a kind of crook)
'assembled this eve' is the wordplay.
'assembled' indicates an anagram.
'this'+'eve'='thiseve'
'thiseve' anagrammed gives 'THIEVES'.
(Other definitions for thieves that I've seen before include "They pinch" , "Having a kitchen" , "Light-fingered people" , "Stealers" , "Robbers" .)