Martial artist's naive punch (5,4)
I believe the answer is:
green belt
'martial artist's' is the definition.
(I've seen this before)
'naive punch' is the wordplay.
'naive' becomes 'green' (similar in meaning).
'punch' becomes 'belt' (belt can mean to punch or hit).
'green'+'belt'='GREEN BELT'
(Other definitions for green belt that I've seen before include "Area of open land around a town where building is restricted" , "status of karate?" , "protected lung" , "Protected land around city" , "area of open land near town" .)