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 see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
The remaining letters 'essen' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'what's' is the 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" .)