Endurance refitting mast in Antarctica, primarily (7)
I believe the answer is:
stamina
'endurance' is the definition.
(I know that stamina is a type of endurance)
'refitting mast in antarctica primarily' is the wordplay.
'refitting' is an anagram indicator.
'primarily' says to take the initial letters.
The first letter of 'antarctica' is 'a'.
'mast' anagrammed gives 'stam'.
'stam'+'in'+'a'='STAMINA'
(Other definitions for stamina that I've seen before include "capability to keep going" , "Ability to maintain effort" , "Endurance, strength to stay the course" , "Sustained energy" , "Indefatigability" .)