Girl describing a run or a martial art (6)
I believe the answer is:
karate
'a martial art' is the definition.
(karate is a kind of martial art)
'girl describing a run' is the wordplay.
'girl' becomes 'kate' (**).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'run' becomes 'r' (cricket abbreviation).
'a'+'r'='ar'
'kate' placed around 'ar' is 'KARATE'.
'or' acts as a link.
(Other definitions for karate that I've seen before include "Martial art (with chopping?)" , "Japanese system of unarmed combat" , "Combative sport" , "Japanese martial art" , "Unarmed-combat system" .)