Fluid melts soap cakes as ultimate bath additive (5,5)
I believe the answer is:
epsom salts
'bath additive' is the definition.
(I've seen this in another clue)
'fluid melts soap cakes as ultimate' is the wordplay.
'fluid' is an anagram indicator.
'cakes' is an insertion indicator (cake can mean to cover).
'ultimate' says to take the final letters (the last or ultimate letter).
The final letter of 'as' is 's'.
'melts'+'soap'='meltssoap'
'meltssoap' is an anagram of 'epomsalts'.
'epomsalts' placed around 's' is 'EPSOM SALTS'.
(Other definitions for epsom salts that I've seen before include "Medicine taken for heartburn and constipation" , "a purge" , "Medicinal preparation" , "Purgative" , "They should clear out!" .)