Ninety splitting fish stand out (5)
I believe the answer is:
excel
'stand out' is the definition.
(I know that stand out can be written as excel)
'ninety splitting fish' is the wordplay.
'ninety' becomes 'XC' (Roman numerals).
'splitting' indicates putting letters inside.
'fish' becomes 'eel' (eel is a type of fish).
'xc' going inside 'eel' is 'EXCEL'.
(Other definitions for excel that I've seen before include "Top" , "Be outstanding" , "Be the best" , "well above average" , "Be outstandingly proficient" .)