Cause of mother's tears? (5)

I believe the answer is:

onion

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'cause of' is the definition.
'onion' can be an answer for 'cause' (I have seen 'Cause of tears' mean 'onion' so perhaps 'cause' could also mean 'onion'). I am not certain of the 'of' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for onion that I've seen before include "associated with 26 [TRIPE] ?" , "Tear-inducing edible bulb" , "The tear-inducing vegetable" , "Strong tasting vegetable" , "veggie" .)

I've seen this clue in the Evening Standard.

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