Courage shown by fool throttling king (4)
I believe the answer is:
grit
'courage' is the definition.
(thesaurus)
'fool throttling king' is the wordplay.
'fool' becomes 'git' (I've seen this in other clues).
'throttling' indicates putting letters inside.
'king' becomes 'r' (r is abbreviation for rex).
'git' enclosing 'r' is 'GRIT'.
'shown by' acts as a link.
(Other definitions for grit that I've seen before include "called for on icy roads" , "Fortitude" , "Indomitable courage" , "Fragments to sprinkle on roads" , "Determination - small stones" .)