Reduction in hostility (4)
I believe the answer is:
thaw
'reduction in hostility' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for thaw that I've seen before include "De-ice" , "Melt; John --, actor" , "Begin to melt" , "Player of detective" , "Morse actor; melt" .)