Harshness of regime is guarantee of unpopular rumbles initially (6)
I believe the answer is:
rigour
'harshness' is the definition.
(both can mean strictness or severity)
'regime is guarantee of unpopular rumbles initially' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'regime is guarantee of unpopular rumbles' is 'RIGOUR'.
'of' acts as a link.
(Other definitions for rigour that I've seen before include "Extreme strictness" , "Quality of being extremely thorough and careful" , "Extreme care and thoroughness" , "Hardship" , "Severity or stringency" .)