Counter is so heartlessly cruel, close to the bone (7)
I believe the answer is:
ossicle
'bone' is the definition.
(I know that ossicle is a type of bone)
'counter is so heartlessly cruel close to the' is the wordplay.
'counter' says the letters should be written in reverse (counter can mean in the opposite direction).
'heartlessly' suggests removing the centre.
'close to' suggests the final letters (the close of something is the end).
'cruel' with its centre taken out is 'cl'.
The last letter of 'the' is 'e'.
'is'+'so'='isso'
'isso' in reverse letter order is 'ossi'.
'ossi'+'cl'+'e'='OSSICLE'
(Other definitions for ossicle that I've seen before include "Very small bone, in the middle ear say" , "Small bone, especially one in the middle ear" , "A hammer" .)