Assertion that mate's tent was broken (9)
I believe the answer is:
statement
'assertion' is the definition.
(I know that assertion can be written as statement)
'mate's tent was broken' is the wordplay.
'mate's tent' anagrams to 'STATEMENT'.
Though, I am unsure how this is indicated.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for statement that I've seen before include "Formal (court) account" , "What bank provides" , "relation" , "may be taken by police" , "It's made, about account say" .)