Animal welfare group appeal for assistance is old hat (7)

I believe the answer is:

petasos

I'm a little stuck... Click here to teach me more about this clue!

'appeal for assistance is old hat' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'animal welfare group' is the wordplay.
'animal' becomes 'pet' (pet is a kind of animal).
'welfare group' becomes 'asos' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pet'+'asos'='PETASOS'

Can you help me to learn more?

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.