Do nothing in extremely leaden town (5)
I believe the answer is:
luton
'town' is the definition.
(I've seen this before)
'do nothing in extremely leaden' is the wordplay.
'do' becomes 'ut' ('ut' can be a synonym of 'do'**).
'nothing' becomes 'o' (looks like zero - 0).
'in' means one lot of letters goes inside another.
'extremely' says to hollow out the word (remove centre letters).
'leaden' with its middle removed is 'ln'.
'ut'+'o'='uto'
'uto' placed into 'ln' is 'LUTON'.
(Other definitions for luton that I've seen before include "Bedfordshire motor and erstwhile millinery town" , "City in Bedfordshire" , "Bedfordshire town and airport" , "Town north of London" , "Beds town" .)