Keep quiet under a tree (3)
I believe the answer is:
ash
'tree' is the definition.
(ash is a kind of tree)
'keep quiet under a' is the wordplay.
'keep quiet' becomes 'sh' (as in shushing someone).
'under' means one lot of letters go next to another.
'sh' put after 'a' is 'ASH'.
(Other definitions for ash that I've seen before include "Silvery grey" , "Tree with winged seeds" , "... for this maybe" , "Wooden" , "End of fire" .)