Insist once more that rare sets are unusual (8)
I believe the answer is:
reassert
'insist once more' is the definition.
('assert' is an answer for 'insist' so perhaps 'reassert' is OK for 'insist once more')
'that rare sets are unusual' is the wordplay.
'that rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
'unusual' indicates an anagram.
'sets'+'are'='setsare'
'setsare' with letters rearranged gives 'eassert'.
'r'+'eassert'='REASSERT'
(Other definitions for reassert that I've seen before include "Again make statement" , "State with force" , "Pronounce once more" , "Make one's presence felt once more" .)