Justification for fighting bombast? (7)
I believe the answer is:
warrant
'justification' is the definition.
'fighting bombast?' is the wordplay.
'fighting' becomes 'war'.
'bombast?' becomes 'rant' (synonyms).
'war'+'rant'='WARRANT'
'for' is the link.
(Other definitions for warrant that I've seen before include "Justify; authorisation" , "Document giving authority" , "Permit for arrest, search of premises, etc" , "Merit, justify" , "Guarantee" .)