Note the unusual power (5)
I believe the answer is:
teeth
'power' is the definition.
('teeth' can metaphorically refer to effectiveness)
'note the unusual' is the wordplay.
'note' becomes 'te' (musical note).
'unusual' indicates an anagram.
'the' is an anagram of 'eth'.
'te'+'eth'='TEETH'
(Other definitions for teeth that I've seen before include "They may be given crowns" , "Structures in the jaws" , "Effective power" , "Incisors maybe" , "The most violent" .)