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