Stew dog starts to devour, eating rapidly (7)
I believe the answer is:
chowder
'stew' is the definition.
(thesaurus)
'dog starts to devour eating rapidly' is the wordplay.
'dog' becomes 'chow' (short for chow chow).
'starts to' indicates taking the first letters.
The initial letters of 'devour eating rapidly' is 'der'.
'chow'+'der'='CHOWDER'
(Other definitions for chowder that I've seen before include "Clam stew, soup" , "Thick fish stew" , "possible first course?" , "Thick seafood soup" , "dish in America" .)