Warn that rate then was different (8)
I believe the answer is:
threaten
'warn' is the definition.
(threatening is a kind of warning)
'rate then was different' is the wordplay.
'was different' is an anagram indicator (I've seen 'is different' mean this).
'rate'+'then'='ratethen'
'ratethen' with letters rearranged gives 'THREATEN'.
'that' is the link.
(Other definitions for threaten that I've seen before include "loom" , "Give warning of something unpleasant" , "Warn of violence" , "Cow" , "Look likely to" .)