Order boxer grain, heartlessly (5)
I believe the answer is:
align
'order' is the definition.
(thesaurus)
'boxer grain heartlessly' is the wordplay.
'boxer' becomes 'Ali' (Muhammad Ali).
'heartlessly' means to remove the middle letters.
'grain' with its middle removed is 'gn'.
'ali'+'gn'='ALIGN'
(Other definitions for align that I've seen before include "Place in a row" , "Ally (oneself with)" , "Arrange so as to be parallel" , "Put in rows" , "Arrange in a row" .)