Boxer's entering the same movement with lofty aspirations (8)
I believe the answer is:
idealism
'movement with lofty aspirations' is the definition.
'boxer's entering the same' is the wordplay.
'boxer's' becomes 'alis' (Muhammad Ali).
'entering' is an insertion indicator.
'the same' becomes 'idem' (Latin for 'the same', used in citations).
'alis' going within 'idem' is 'IDEALISM'.
(Other definitions for idealism that I've seen before include "Noble attitude from miladies" , "Pursuit of the perfect" , "Thoughts of perfection for miladies" , "Aspirations to good things for miladies" , "Mental state of seeking perfection or highest standards" .)