Soap additive (4)
I believe the answer is:
aloe
'soap additive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aloe that I've seen before include "Plant source of purgative drug" , "Genus of bitter herbs" , "Bitter drug from a leaf juice" , "shrub" , "O, ale with bitter plant" .)