Outstanding figures reportedly make firm leader in sector (6)
I believe the answer is:
titans
'outstanding figures' is the definition.
(outstanding individuals in a field)
'reportedly make firm leader in sector' is the wordplay.
'reportedly' shows a homophone (sound like).
'make firm' becomes 'tighten' (tighten can mean to make firm or hard).
'leader in' says to take the initial letters.
The initial letter of 'sector' is 's'.
'tighten' sounds like 'titan'.
'titan'+'s'='TITANS'
(Other definitions for titans that I've seen before include "Old Greek gods, very strong like Atlas" , "Taints found in great men" , "Elder deities of Greek myth" , "Mythological beings of great size and strength" , "Taints in mythical giants" .)