Crossword Genius

Bubbly effect coming to a halt on a half of beer? (6)

Ross

I believe the answer is:

lather

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

'bubbly effect coming' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'halt on a half of beer?' is the wordplay.
I cannot really understand how this works, but
an anagram of 'halt' is 'lath' which is present in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'to a' is the link.

Can you help me to learn more?

(Other definitions for lather that I've seen before include "Froth, thrash (colloq.)" , "Frothy bubbles from soap" , "Foam - tizzy (informal)" , "Turner" , "Agitated state" .)

I've seen this clue in the Evening Standard.
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