Boxer with extremely capable girl (5)
I believe the answer is:
alice
'girl' is the definition.
'boxer with extremely capable' is the wordplay.
'boxer' becomes 'ali' (Muhammad Ali).
'with' says to put letters next to each other.
'extremely' means to remove the middle letters.
'capable' with its middle taken out is 'ce'.
'ali'+'ce'='ALICE'
(Other definitions for alice that I've seen before include "Immortal girl in Wonderland" , "Wonderland girl's name" , "Immortal girl created by Lewis Carroll" , "Miss Liddell" , "Wonderland heroine" .)