Shaving brushes ultimately turn bad and start to rot (6)
I believe the answer is:
sliver
'shaving' is the definition.
(thesaurus)
'brushes ultimately turn bad and start to rot' is the wordplay.
'ultimately' indicates one should take the final letters.
'turn' says the letters should be written in reverse.
'bad' becomes 'evil' ('evil' is associated in meaning with 'bad'**).
'and' says to put letters next to each other.
'start to' suggests taking the first letters.
The last letter of 'brushes' is 's'.
The initial letter of 'rot' is 'r'.
'evil' back-to-front is 'live'.
's'+'live'+'r'='SLIVER'
(Other definitions for sliver that I've seen before include "Sharp piece (of glass)" , "Shard" , "Wafer-thin slice" , "Thin fragment or slice" , "Piece cut off" .)