Mysterious thing appearing regularly in outflow (3)
I believe the answer is:
ufo
'mysterious thing' is the definition.
'regularly in outflow' is the wordplay.
'regularly in' means one should take alternating letters.
The alternate letters of 'outflow' are 'UFO'.
'appearing' acts as a link.
(Other definitions for ufo that I've seen before include "Initially and formally, flying saucer" , "Potential alien ship" , "Potential alien craft" , "Strange sighting in the sky, in short" , "ET ship" .)