Where you could picnic and crack some walnuts (4)
I believe the answer is:
lawn
'where you could picnic and crack some walnuts' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'where you could picnic and' is the wordplay.
I cannot really understand how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
The remaining letters 'law' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lawn that I've seen before include "Expanse of grass" , "Cultivated and mowed grass" , "material" , "that may be cut in strips?" , "type of tennis" .)