Fallen angel's striking if cruel (7)
I believe the answer is:
lucifer
'fallen angel's' is the definition.
(I've seen this in another clue)
'striking if cruel' is the wordplay.
'striking' indicates an anagram ('strike' can mean 'hit' or 'disrupt').
'if'+'cruel'='ifcruel'
'ifcruel' is an anagram of 'LUCIFER'.
(Other definitions for lucifer that I've seen before include "flares up when struck" , "from the abyss?" , "Match - Satan" , "Rebellious angel; a match" , "Venus" .)