Dad rants about flag (8)
I believe the answer is:
standard
'flag' is the definition.
(I know that flag can be written as standard)
'dad rants about' is the wordplay.
'about' indicates an anagram.
'dad'+'rants'='dadrants'
'dadrants' anagrammed gives 'STANDARD'.
(Other definitions for standard that I've seen before include "type of flag" , "Flying colours" , "sort of lamp" , "Flag - pattern - guide" , "'Reference point, touchstone (8)'" .)