Seen briefly (8)
I believe the answer is:
glimpsed
'seen briefly' is the definition.
('glimpse' can be a synonym of 'see briefly')
This is all the clue.
(Other definitions for glimpsed that I've seen before include "Caught quick sight of" , "Caught momentary sight of" , "Saw briefly" , "Saw fleetingly" , "Seen briefly" .)