A labyrinth briefly seen on river (6)
I believe the answer is:
amazon
'river' is the definition.
(Amazon River is an example)
'a labyrinth briefly seen on' is the wordplay.
'labyrinth' becomes 'maze' (synonyms).
'briefly' means to remove the last letter.
'seen' means one lot of letters go next to another.
'maze' with its final letter removed is 'maz'.
'a'+'maz'+'on'='AMAZON'
(Other definitions for amazon that I've seen before include "Flower" , "tropical fish swimming here" , "Strong woman - large river" , "One of a mythic nation of women warriors" , "seller on the Internet" .)