Untidy person's thorough wash mostly very loud (6)
I believe the answer is:
scruff
'untidy person's' is the definition.
(I've seen this before)
'thorough wash mostly very loud' is the wordplay.
'thorough wash' becomes 'scrub' (scrubbing is washing thoroughly).
'mostly' means to remove the last letter.
'very loud' becomes 'ff' (musical abbreviation).
'scrub' with its final letter removed is 'scru'.
'scru'+'ff'='SCRUFF'
(Other definitions for scruff that I've seen before include "Back of the neck; untidy person" , "Nape; untidy type" , "Part of neck" , "Dirty, ill-dressed person" , "One hardly washing" .)