Dog chasing down butterfly (11)
I believe the answer is:
swallowtail
'butterfly' is the definition.
(I've seen this before)
'dog chasing down' is the wordplay.
'dog' becomes 'tail' (synonyms).
'chasing' means one lot of letters go next to another (some letters chase after others).
'down' becomes 'swallow' (both can mean to consume or eat).
'tail' after 'swallow' is 'SWALLOWTAIL'.
(Other definitions for swallowtail that I've seen before include "coat" , "Butterfly" , "insect" , "In circle, mythical snake does" .)