After half a mile there's a change in the heart beat (7)
I believe the answer is:
leather
'beat' is the definition.
(to leather can mean to beat or thrash)
'after half a mile there's a change in the heart' is the wordplay.
'after' means one lot of letters go next to another.
'half a' indicates taking half.
'there's a change in' is an anagram indicator.
'mile' with half the letters taken is 'le'.
'heart' anagrammed gives 'ather'.
'le'+'ather' is 'LEATHER'.
(Other definitions for leather that I've seen before include "Processed animal skin" , "Flog; hide" , "calf" , "Tanned animal hide" , "Material made from animal skin" .)