Warn one then about the rate of exchange (8)
I believe the answer is:
threaten
'warn' is the definition.
(threatening is a kind of warning)
'one then about the rate of exchange' is the wordplay.
'about' means one lot of letters goes inside another.
'of exchange' indicates anagramming the letters (I've seen 'exchange' mean this).
'rate' is an anagram of 'reat'.
'then' enclosing 'reat' is 'THREATEN'.
(Other definitions for threaten that I've seen before include "State an intention to harm" , "indicate danger" , "loom" , "Foreshadow" , "Cow" .)