The length a boxer will go to (5)
I believe the answer is:
reach
'go to' is the definition.
(thesaurus)
'the length a boxer' is the wordplay.
'the length' becomes 'r' (r is a kind of length).
'a boxer' becomes 'each' (I can't justify this - if you can you should believe this answer much more).
'r'+'each'='REACH'
'will' acts as a link.
Can you help me to learn more?
(Other definitions for reach that I've seen before include "Arrive at a point" , "It's an advantage for a boxer" , "stretch of canal between locks" , "bay" , "Hands up" .)