The Dome could never quite have been built by this system (6)
I believe the answer is:
method
'this system' is the definition.
(I've seen this before)
'the dome could never quite have been' is the wordplay.
I cannot really see how this works, but
an anagram of 'the' is 'eth' which is within the answer.
The remaining letters 'mod' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'built by' acts as a link.
Can you help me to learn more?
(Other definitions for method that I've seen before include "Way of doing something, technique" , "Way of doing something, procedure" , "Fashion" , "Style of technique, as of some actors" , "rule" .)