Principal lover cut knot occasionally (9)
I believe the answer is:
paramount
'principal' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for paramount that I've seen before include "Superior to all others" , "Supreme - Hollywood studio" , "outstanding" , "Of the greatest importance" , "Of overriding importance" .)