Doctor with duck in brook shows what is fundamental (7)
I believe the answer is:
bedrock
'what is fundamental' is the definition.
(bedrock means something fundamental)
'doctor with duck in brook' is the wordplay.
I cannot quite understand how this works, but
'doctor' could be 'dr' (abbreviation) and 'dr' is present in the answer.
'duck' could be 'o' (resembles zero - 'duck' in cricket) and 'o' is present in the answer.
The remaining letters 'beck' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'shows' is the link.
(Other definitions for bedrock that I've seen before include "Basis" , "Fundamental principles" , "Fundamental principle" , "Solid foundation" .)