Dog chewed up dad's unopened lunch, swallowing hard (9)
I believe the answer is:
'dog' is the definition.
(I know that Dachshund is a type of dog)
'chewed up dad's unopened lunch swallowing hard' is the wordplay.
'chewed up' indicates an anagram.
'unopened' means to remove the first letter (without opening letter).
'swallowing' means one lot of letters goes inside another.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'lunch' with its first letter removed is 'unch'.
'dadsunch' is an anagram of 'dacshund'.
'dacshund' placed around 'h' is 'DACHSHUND'.
(Other definitions for dachshund that I've seen before include "Long, low dog" , "Dad's hunch about the dog" , "Small dog (from Germany?)" , "Long-bodied, short-legged breed of dog" , "Sausage 11" .)