Unflinching (6)
(Other definitions for steely that I've seen before include "As hard, or grey, as metal" , "Inflexibly severe" , "Hard and metallic" , "Ruthless" , "Gimlet-eyed" .)
'unflinching' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stoic that I've seen before include "Athenian philosopher" , "Phlegmatic type" , "Uncomplaining in suffering" , "Someone seemingly indifferent to pleasure or pain" , "Philosopher like Marcus Aurelius" .)