Was emphatic that the side isn’t to be beaten (8)
I believe the answer is:
insisted
'was emphatic' is the definition.
(I've seen this before)
'the side isn't to be beaten' is the wordplay.
'to be beaten' is an anagram indicator.
'side'+'isnt'='sideisnt'
'sideisnt' is an anagram of 'INSISTED'.
'that' is the link.
(Other definitions for insisted that I've seen before include "Declared emphatically about tidiness" , "Stated positively and assertively" , "Declared or demanded emphatically" , "... repeated" , "Stated emphatically, refusing to budge" .)