Crossword Genius

Diminish what’s left at Big Bertha’s place (6)

Ross

I believe the answer is:

lessen

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

'diminish' is the definition.
(synonyms)

'left at big bertha's place' is the wordplay.
I cannot quite understand how this works, but
'left' could be 'l' (common abbreviation) and 'l' is located in the answer.
The remaining letters 'essen' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'what's' acts as a link.

Can you help me to learn more?

(Other definitions for lessen that I've seen before include "Become smaller or fewer" , "Abate" , "Contract" , "Shrink" , "Cut back or diminish" .)

I've seen this clue in the Evening Standard.
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