Diminish what’s left at Big Bertha’s place (6)
I believe the answer is:
lessen
'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" .)