Filthy dog kept in small yard (6)
I believe the answer is:
smutty
'filthy' is the definition.
(thesaurus)
'dog kept in small yard' is the wordplay.
'dog' becomes 'mutt' (mutt is a kind of dog**).
'kept in' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'yard' becomes 'y'.
's'+'y'='sy'
'mutt' inserted within 'sy' is 'SMUTTY'.
(Other definitions for smutty that I've seen before include "Blue" , "Dirty" , "Rather indecent" , "Rather rude" , "Filthy" .)