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