Feudal drudge returns with mop that's not found in big cities (5,3)
I believe the answer is:
fresh air
'not found in big cities' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'feudal drudge returns with mop' is the wordplay.
'feudal drudge' becomes 'serf' (I've seen this in another clue).
'returns' is a reversal indicator.
'with' is a charade indicator (letters next to each other).
'mop' becomes 'hair' (I can't explain this - if you can you should give a lot more credence to this answer).
'serf' back-to-front is 'fres'.
'fres'+'hair'='FRESH AIR'
'that's' is the link.
Can you help me to learn more?
(Other definitions for fresh air that I've seen before include "that's found outside" , "Type of fiend" .)