Flabbergast with one labyrinth (5)
I believe the answer is:
amaze
'flabbergast' is the definition.
(I've seen this in another clue)
'with one labyrinth' is the wordplay.
'with' is a charade indicator (letters next to each other).
'one' becomes 'a' (a thing is one thing).
'labyrinth' becomes 'maze' (synonyms).
'a' put next to 'maze' is 'AMAZE'.
(Other definitions for amaze that I've seen before include "Bowl over" , "Startle" , "Floor" , "Greatly surprise, fill with wonder" , "Stagger" .)