Brook runs into vapour (6)
I believe the answer is:
stream
'brook' is the definition.
(I've seen this before)
'runs into vapour' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' means one lot of letters goes inside another.
'vapour' becomes 'steam' (steam is a kind of vapour).
'r' put into 'steam' is 'STREAM'.
(Other definitions for stream that I've seen before include "Brook or small river" , "Steady flow; pupil group" , "Small watercourse" , "Small narrow river" , "'Brook, runnel (6)'" .)