Wave altered and hit bluff (4,6)
I believe the answer is:
'bluff' is the definition.
(an idle threat is a bluff)
'wave altered and hit' is the wordplay.
'wave' indicates an anagram (I've seen 'waving' mean this (letters wave around)).
'and' is a charade indicator (letters next to each other).
'altered' after 'hit' is 'hitaltered'.
'hitaltered' is an anagram of 'IDLE THREAT'.
(Another definition for idle threat that I've seen is " Unconvincing menace".)