Woolgathering (7)
'woolgathering' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for reverie that I've seen before include "Daydream or preoccupation" , "A dreamlike state, daydream" , "I revere in daydream" , "Aimless train of thought" , "Daydream, meditation" .)
'woolgathering' is the definition.
(thesaurus)
This is all the clue.