Run-down spot, extremely dirty (5)
I believe the answer is:
seedy
'run-down' is the definition.
(I've seen this before)
'spot extremely dirty' is the wordplay.
'spot' becomes 'see' (I've seen this in another clue).
'extremely' suggests removing the centre (only the extremes of the word are used).
'dirty' with its centre taken out is 'dy'.
'see'+'dy'='SEEDY'
(Other definitions for seedy that I've seen before include "Run-down, squalid" , "Sordid and disreputable" , "Sleazy, sordid" , "Unseemly" , "Unpleasant" .)