Want to put a blemish on a large town (8)
I believe the answer is:
scarcity
'want' is the definition.
(I've seen this before)
'a blemish on a large town' is the wordplay.
'a blemish' becomes 'scar' (I've seen this in another clue).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'a large town' becomes 'city' (a city is a large settlement).
'scar'+'city'='SCARCITY'
'to put' is the link.
(Other definitions for scarcity that I've seen before include "Want" , "Thinness" , "Something uncommon" , "Inadequate supply" , "Poverty" .)