Boxer’s in mire over philosophical principle (7)
I believe the answer is:
dualism
'philosophical principle' is the definition.
(dualism is a philosophical idea)
'boxer's in mire over' is the wordplay.
'boxer's' becomes 'Ali's' (Muhammad Ali).
'in' indicates putting letters inside.
'mire' becomes 'mud' (synonyms).
'over' shows that the letters should be reversed in order.
'mud' backwards is 'dum'.
'alis' inserted into 'dum' is 'DUALISM'.
(Other definitions for dualism that I've seen before include "Eg, what Yin and Yang represent" , "Division into two opposed aspects" , "Belief in two basic entities" .)