One guy - mad - frequently found in public smoking area (5,3)
I believe the answer is:
opium den
'smoking area' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'one guy mad frequently found in public' is the wordplay.
'one' becomes 'i' (Roman numeral).
'frequently' means one should take alternating letters (letters selected at a regular frequency).
'found in' indicates putting letters inside.
'public' becomes 'open' (similar in meaning).
'guy'+'mad'='guymad'
The alternating letters of 'guymad' are 'umd'.
'i'+'umd'='iumd'
'iumd' put inside 'open' is 'OPIUM-DEN'.
Can you help me to learn more?
(Other definitions for opium den that I've seen before include "smoke-filled room" , "Place to buy and smoke dope" , "place for addicts" , "Place to take drugs" .)