Rolling maul involves stream and tree (8)
I believe the answer is:
laburnum
'tree' is the definition.
(type of European tree)
'rolling maul involves stream' is the wordplay.
'rolling' is an anagram indicator.
'involves' means one lot of letters goes inside another.
'stream' becomes 'burn' (a burn is a small stream).
'maul' anagrammed gives 'laum'.
'laum' going around 'burn' is 'LABURNUM'.
'and' is the link.
(Other definitions for laburnum that I've seen before include "Poisonous shrub with yellow flowers" , "Tree with yellow flowers and poisonous seeds" , "Ornamental tree with hanging clusters of yellow flowers" , "Poisonous flowering shrub - run album (anag)" , "Poisonous plant" .)