What is soothing in herbal mixture (4)
I believe the answer is:
balm
'what is soothing' is the definition.
'in herbal mixture' is the wordplay.
'in' indicates the answer is hidden within the clue.
'BALM' is hidden in the letters of 'herbal mixture'.
(Other definitions for balm that I've seen before include "A soothing influence on lamb" , "Something soothing" , "that will soothe the skin" , "Soothing ointment for lamb (4)" , "Soothing medicine for lamb" .)