Small cleansing plant (6)
I believe the answer is:
spurge
'plant' is the definition.
(spurge is a shrub)
'small cleansing' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cleansing' becomes 'purge' (purge is a kind of cleansing).
's'+'purge'='SPURGE'
(Other definitions for spurge that I've seen before include "Plant of Euphorbia genus" , "Type of shrub" .)