Devil is cruel if disturbed (7)
I believe the answer is:
lucifer
'devil' is the definition.
('Lucifer' can be a synonym of 'devil')
'cruel if disturbed' is the wordplay.
'disturbed' indicates an anagram.
'cruel'+'if'='cruelif'
'cruelif' with letters rearranged gives 'LUCIFER'.
'is' is the link.
(Other definitions for lucifer that I've seen before include "Morning star" , "Light" , "Satan, Son of the Morning" , "leader of rebellion" , "Devil - match!" .)