Overachiever, abandoning reverie, unleashed chaos (5)
I believe the answer is:
havoc
'chaos' is the definition.
(I've seen this before)
'overachiever abandoning reverie unleashed' is the wordplay.
'abandoning' indicates named letters should be taken away.
'unleashed' indicates an anagram (letters are unleashed from their original order).
'overachiever' with 'reverie' taken away is 'oachv'.
'oachv' with letters rearranged gives 'HAVOC'.
(Other definitions for havoc that I've seen before include "desolation" , "Mayhem" , "Ruin" , "Violent and needless disturbance" , "Chaos and destruction" .)