Numbing cold crushing Scott's heart, Amundsen ultimately coming first (8)
I believe the answer is:
narcotic
'numbing' is the definition.
(I've seen this before)
'cold crushing scott's heart amundsen ultimately coming first' is the wordplay.
'cold' becomes 'arctic' (similar in meaning).
'crushing' indicates putting letters inside.
'heart' says to take the centre.
'ultimately' says to take the final letters.
'coming first' is a charade indicator (letters next to each other).
The middle of 'scott' is 'o'.
The final letter of 'amundsen' is 'n'.
'arctic' enclosing 'o' is 'arcotic'.
'arcotic' after 'n' is 'NARCOTIC'.
(Other definitions for narcotic that I've seen before include "Addictive drug affecting mood or behaviour" , "Drug that induces sleep" , "Stupor-inducing drug" , "Drug that induces drowsiness or stupor" , "Drug to induce drowsiness or stupor" .)