Tough and mean boxing king (7)
I believe the answer is:
stringy
'tough' is the definition.
(synonyms)
'mean boxing king' is the wordplay.
'mean' becomes 'stingy' (both can mean miserly).
'boxing' is an insertion indicator ('box' can mean contain, as in to put in a box).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'stingy' going around 'r' is 'STRINGY'.
'and' acts as a link.
(Other definitions for stringy that I've seen before include "Tall, wiry, thin" , "Old runners can be so" , "(Of food) tough, fibrous" .)