Dead level (4,3,4)
(Other definitions for neck and neck that I've seen before include "Absolutely level (in race)" , "Bill and coo" , "New lovers frequently do it" , "side by side" , "virtually inseparable" .)
I believe this is a double definition.
'dead' is the first definition.
(thesaurus)
'level' is the second definition.
(synonyms)
(Other definitions for flat that I've seen before include "Prostrate" , "Level; dull" , "Digs perhaps" , "Dull; set of rooms" , "Apartment; plane" .)