Crossword Genius

Is one permitted to catch Welsh lady from the south dressing? (10)

Ross

I believe the answer is:

mayonnaise

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

'dressing?' is the definition.
(I know that mayonnaise is a type of dressing)

'is one permitted to catch welsh lady from the south' is the wordplay.
I cannot really understand how this works, but
'is' is found in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
'from' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for mayonnaise that I've seen before include "Salad dressing" , "Egg/oil sauce" , "Creamy dressing" , "7 for salad" , "which is saucy" .)

I've seen this clue in The Guardian.
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