It's feasible dealing with with boxer's idiosyncrasy (9)
I believe the answer is:
realistic
'it's feasible' is the definition.
(thesaurus)
'dealing with with boxer's idiosyncrasy' is the wordplay.
'dealing with' becomes 're' (regarding).
'with' is a charade indicator (letters next to each other).
'boxer's' becomes 'Ali's' (Muhammad Ali).
'idiosyncrasy' becomes 'tic' (someone's tics are their idiosyncrasies or habits).
're'+'alis'+'tic'='REALISTIC'
(Other definitions for realistic that I've seen before include "Objective" , "true-to-life" , "Without illusions, dealing in facts" , "It is clear, being practical and sensible" , "believable" .)