Washes Usain Bolt's pants (9)
I believe the answer is:
ablutions
'washes' is the definition.
(ablution is a kind of wash)
'usain bolt's pants' is the wordplay.
'pants' indicates an anagram (UK informal term for 'nonsense' or 'rubbish').
'usain'+'bolt'='usainbolt'
'usainbolt' with letters rearranged gives 'ABLUTIONS'.
(Other definitions for ablutions that I've seen before include "wash and brush-up" , "The act of washing" .)