Fundamentally a threat is revealed (2,5)
I believe the answer is:
at heart
'fundamentally' is the definition.
(I've seen this before)
'a threat is revealed' is the wordplay.
'is revealed' indicates anagramming the letters.
'threat' anagrammed gives 'theart'.
'a'+'theart'='AT HEART'
(Other definitions for at heart that I've seen before include "Really" , "A threat, in one's inmost feelings" , "In reality" , "Substantially" , "Fundamentally" .)