Shoot rook in stream (6)
I believe the answer is:
sprout
'shoot' is the definition.
(I know that shoot can be written as sprout)
'rook in stream' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'in' means one lot of letters goes inside another.
'stream' becomes 'spout' (both can mean to emit liquid).
'r' inserted inside 'spout' is 'SPROUT'.
(Other definitions for sprout that I've seen before include "Small cabbage from Brussels?" , "Start to grow" , "Put out shoots for the vegetable" , "Grow like Proust" , "Suddenly grow" .)