Very dirty (5)
(Other definitions for mucky that I've seen before include "Covered in dirt" , "Really dirty, filthy" .)
(Other definitions for grimy that I've seen before include "Mucky" , "filthy" , "Dirty - soiled" , "Besmirched" , "smutty" .)
'very dirty' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for filthy that I've seen before include "Mucky" , "Dirty - smutty" , "Really dirty, disgusting" , "to be so rich?" , "Offensive" .)