Where you could picnic and crack some walnuts (4)
I believe the answer is:
lawn
'where you could picnic' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'crack some walnuts' is the wordplay.
'crack' becomes 'law' (I can't explain this - if you can you should give a lot more credence to this answer).
'some' says to take the centre.
The central letter of 'walnuts' is 'n'.
'law'+'n'='LAWN'
'and' acts as a link.
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" .)