Soothing substance (4)
'soothing substance' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for balm that I've seen before include "Royal holiday residence on the River Dee" , "It's comforting" , "Soothing stuff for lamb" , "Soothing medicine for lamb" , "Soothing herb" .)
(Other definitions for unguent that I've seen before include "Lubricant to tune gun" , "Lotion" , "Balm, ointment" , "An ointment or lubricant" , "Ointment, soothing salve" .)