Dog leads on many, unlikely terriers tethered (4)
I believe the answer is:
mutt
'dog' is the definition.
(I know that mutt is a type of dog)
'leads on many unlikely terriers tethered' is the wordplay.
'leads on' suggests taking the first letters (the letters which lead the words).
The initial letters of 'many unlikely terriers tethered' is 'MUTT'.
(Other definitions for mutt that I've seen before include "Setter perhaps" , "Thematic type [DOG]" , "far from best in show?" , "Chump" , "Mongrel dog - foolish person" .)