Essentially a nasty threat (2,5)
I believe the answer is:
at heart
'essentially' is the definition.
(I've seen this before)
'a nasty threat' is the wordplay.
'nasty' indicates anagramming the letters.
'threat' anagrammed gives 'theart'.
'a'+'theart'='AT HEART'
(Other definitions for at heart that I've seen before include "In one's innermost feelings, in that era" , "Substantially" , "In one's real nature - a threat (anag)" , "In reality" , "real character" .)