Lacking sleep, son leaves city (6)
I believe the answer is:
naples
'city' is the definition.
(Naples is an example)
'lacking sleep son' is the wordplay.
'lacking' means to remove the last letter (the word is incomplete or lacking).
'sleep' becomes 'nap' (a nap is a short sleep).
'son' becomes 'ples' (I can't explain this - if you can you should give a lot more credence to this answer).
'nap' with its final letter taken away is 'na'.
'na'+'ples'='NAPLES'
'leaves' is the link.
Can you help me to learn more?
(Other definitions for naples that I've seen before include "Italian port blockaded by Nelson" , "European city" , "Port near to Vesuvius" , "Planes go awry in Italian port" , "Visit here before you go" .)