Mice emptied place for grain left over in London, say (10)
I believe the answer is:
metropolis
'london say' is the definition.
(London is an example)
'mice emptied place for grain left over' is the wordplay.
'emptied' suggests removing the centre.
'place for grain' becomes 'silo' (store for grain on a farm).
'left' becomes 'port' (left side of a ship - opposite of starboard).
'over' says the letters should be written backwards.
'mice' with its centre removed is 'me'.
'silo'+'port'='siloport'
'siloport' backwards is 'tropolis'.
'me'+'tropolis'='METROPOLIS'
'in' acts as a link.
(Other definitions for metropolis that I've seen before include "capital" , "population centre" , "chief centre" , "Main city" , "Very large city" .)