Flow of vapour across river (6)
I believe the answer is:
stream
This could be a double definition.
'flow of vapour' is the first definition.
'stream' can be an answer for 'flow' (synonyms). I'm not certain of the 'of vapour' bit.
'across river' is the second definition.
'stream' can be an answer for 'river' (I've seen this in another clue). I'm unsure of the 'across' bit.
Can you help me to learn more?
(Other definitions for stream that I've seen before include "Fly" , "Rill, brook" , "'Brook, runnel (6)'" , "Steady flow; pupil group" , "group of schoolchildren" .)