Do leave off the Dome as a topic! (5)
I believe the answer is:
theme
'a topic' is the definition.
(I know that topic can be written as theme)
'do leave off the dome' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
The remaining letters 'heme' 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).
'as' is the link.
Can you help me to learn more?
(Other definitions for theme that I've seen before include "Melody on which variations are constructed" , "Topic; melody" , "issue" , "Subject (of talk, text, etc)" , "sort of tune" .)