Leading thinker's work on female brilliance is outstanding (3-6)
I believe the answer is:
top-flight
'outstanding' is the definition.
(I've seen this before)
'leading thinker's work on female brilliance' is the wordplay.
'leading' indicates taking the first letters.
'work' becomes 'op' (abbreviation for opus).
'on' says to put letters next to each other.
'female' becomes 'f' (abbreviation).
'brilliance' becomes 'light' (brilliant can mean shining or lit up).
The first letter of 'thinkers' is 't'.
't'+'op'+'f'+'light'='TOP-FLIGHT'
'is' is the link.
(Other definitions for top-flight that I've seen before include "Superior" , "Excellent -- best possible" , "Highest level" , "Of the highest class" .)