Legitimate knockout finishes off contender before middle of the second round (6)
I believe the answer is:
kosher
'legitimate' is the definition.
(thesaurus)
'knockout finishes off contender before middle of the second round' is the wordplay.
'knockout' becomes 'ko' ('KO' can be a synonym of 'knockout').
'finishes off' suggests the final letters (take the finishing letters off of the words).
'before' means one lot of letters go next to another.
'middle of the second' becomes 'ehs' (I can't justify this - if you can you should give a lot more credence to this answer).
'round' is a reversal indicator.
The last letter of 'contender' is 'r'.
'r'+'ehs'='rehs'
'rehs' back-to-front is 'sher'.
'ko'+'sher'='KOSHER'
Can you help me to learn more?
(Other definitions for kosher that I've seen before include "strict rules" , "Edible for Jew" , "Legitimate (according to Jewish laws)" , "Pure (Jewish)" , "Correct ? legitimate" .)