Highest kind of fractions (7)
I believe the answer is:
eighths
'fractions' is the definition.
(eighth is a kind of fraction)
'highest kind of' is the wordplay.
'kind of' is an anagram indicator.
'highest' with letters rearranged gives 'EIGHTHS'.
(Other definitions for eighths that I've seen before include "Half quarters" , "Parts of division" , "Fractions of 12? per cent" .)