A thrash in large town is a disaster (8)
I believe the answer is:
calamity
'is a disaster' is the definition.
(I've seen this before)
'a thrash in large town' is the wordplay.
'thrash' becomes 'lam' (I've seen this before).
'in' is an insertion indicator.
'large town' becomes 'city' (I've seen this before).
'a'+'lam'='alam'
'alam' put inside 'city' is 'CALAMITY'.
(Other definitions for calamity that I've seen before include "Event resulting in great loss and misfortune" , "distress" , "Tragedy" , "Disaster, sudden great misfortune" , "Musical set in Deadwood" .)