To dad, it's never-ending nonsense you mindlessly repeat (6)
I believe the answer is:
parrot
'mindlessly repeat' is the definition.
(to parrot something is to repeat it)
'to dad it's never-ending nonsense you' is the wordplay.
'to dad' becomes 'pa' (both can mean one's father).
'ending' says to take the final letters (the ending of the word).
'nonsense you' becomes 'rot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The final letter of 'never' is 'r'.
'pa'+'r'+'rot'='PARROT'
Can you help me to learn more?
(Other definitions for parrot that I've seen before include "Vividly coloured bird" , "Mimic speech like colourful bird" , "Tropical bird for Pat Orr" , "Brightly coloured tropical bird" , "A flighty creature" .)