Make water stay under small flower (9)
I believe the answer is:
speedwell
'flower' is the definition.
(speedwell is a kind of flower)
'make water stay under small' is the wordplay.
'make water' becomes 'pee' ('pee' can be a synonym of 'make water').
'stay' becomes 'dwell' (I've seen this before).
'under' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pee'+'dwell'='peedwell'
'peedwell' after 's' is 'SPEEDWELL'.
(Other definitions for speedwell that I've seen before include "Small blue flower of the genus veronica" , "Flowering plant" , "Small plant, veronica" , "Small creeping herbaceous plant with blue or pink flowers" , "bloomer?" .)