Pure heart possessed by beaten Masai warrior (7)
I believe the answer is:
samurai
'warrior' is the definition.
(samurai is a kind of warrior)
'pure heart possessed by beaten masai' is the wordplay.
'heart' indicates the central letters.
'possessed by' means one lot of letters goes inside another.
'beaten' indicates anagramming the letters.
The middle letters of 'pure' are 'ur'.
'masai' anagrammed gives 'samai'.
'ur' inserted inside 'samai' is 'SAMURAI'.
(Other definitions for samurai that I've seen before include "Trad. Japanese warrior caste" , "Sudoku variant" , "Japanese militarist" , "Japanese fighter" , "Japanese warrior code" .)