Where, at Hampstead, you can dine in short pants
I believe the answer is:
heath
'where at hampstead you' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'dine in short pants' is the wordplay.
'dine' becomes 'eat' (I've seen this before).
'in' indicates putting letters inside.
'short pants' becomes 'hh' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'eat' going into 'hh' is 'HEATH'.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for heath that I've seen before include "Sandy, scrubby land" , "Tract of open land" , "Edward -, former PM" , "Area of open land with gorse etc" , "Ex-Tory PM" .)