Dad's rants about principles (9)
I believe the answer is:
standards
'principles' is the definition.
(I've seen this before)
'dad's rants about' is the wordplay.
'about' is an anagram indicator.
'dads'+'rants'='dadsrants'
'dadsrants' is an anagram of 'STANDARDS'.
(Other definitions for standards that I've seen before include "Principles of integrity" , "Touchstones, reference points" , "Flags for bases of measurement" , "Principles of decent behaviour" , "Flags; levels of quality" .)