Cruel act makes a rot settle over town (8)
I believe the answer is:
atrocity
'cruel act' is the definition.
'atrocity' can be an answer for 'act' (atrocity is a kind of act). I am not sure about the 'cruel' bit.
'a rot settle over town' is the wordplay.
'settle' indicates an anagram (I've seen 'settled' mean this).
'over' is an insertion indicator.
'town' becomes 'city' (I have seen 'Large town' mean 'city' so perhaps 'town' could also mean 'city').
'rot' is an anagram of 'tro'.
'a' going inside 'tro' is 'atro'.
'atro'+'city'='ATROCITY'
'makes' is the link.
Can you help me to learn more?
(Other definitions for atrocity that I've seen before include "Barbarity, inhumanity" , "Barbarous, inhuman act" , "Cruel deed" , "Heinous deed" , "Frightful act of citatory kind" .)