Bit soft thing (8)
I believe the answer is:
particle
'bit' is the definition.
(I have seen 'Small bit ' mean 'particle' so perhaps 'bit' could also mean 'particle')
'soft thing' is the wordplay.
'soft' becomes 'p' (musical abbreviation for piano, 'soft').
'thing' becomes 'article' (I've seen this before).
'p'+'article'='PARTICLE'
(Other definitions for particle that I've seen before include "Fragment of matter" , "Prelatic (anag.)" , "may be just a speck of dust" , "Tiny piece of anything" , "Tiny piece of matter" .)