Rational boxer inspired by others in charge (9)
I believe the answer is:
realistic
'rational' is the definition.
(thesaurus)
'boxer inspired by others in charge' is the wordplay.
'boxer' becomes 'Ali' (Muhammad Ali).
'inspired by' is an insertion indicator (some letters breathed in or 'inspired' by others).
'others' becomes 'rest' (eg the rest of a group).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'ali' going within 'rest' is 'realist'.
'realist'+'ic'='REALISTIC'
(Other definitions for realistic that I've seen before include "Pragmatic" , "Facing facts, as it is clear" , "Faithful" , "accurate picture" , "Objective" .)