There’s no doubt about it, you’re off key there! (5,4)
I believe the answer is:
thats flat
'there's no doubt' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'about it you're off key there' is the wordplay.
I cannot really understand how this works, but
'about' could be 'a' (abbreviation in Chambers) and 'a' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
'key' could be 'f' (musical key) and 'f' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for thats flat that I've seen before include "there's nothing more to be said" , "... may provoke this criticism" , "There's no argument" , "Informally, it's quite definite - or horizontal?" , "lacks sparkle" .)