Due to brittle edges, join with justification (7)
I believe the answer is:
because
'due' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'brittle edges join with justification' is the wordplay.
'brittle edges join' becomes 'be' (I can't explain this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'justification' becomes 'cause' (cause is a kind of justification).
'be'+'cause'='BECAUSE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for because that I've seen before include "Thanks to this" , "Beginning to explain" , "Owing to" , "Since" , "For the reason that" .)