By hacking, clear top of the chest (8)
I believe the answer is:
pectoral
'chest' is the definition.
(I've seen this before)
'by hacking clear top of the' is the wordplay.
'by' becomes 'p' (I can't explain this - if you can you should give a lot more credence to this answer).
'hacking' is an anagram indicator.
'top' suggests taking the first letters (in a down clue, the first letter is top).
The initial letters of 'of the' is 'ot'.
'clear'+'ot'='clearot'
'clearot' with letters rearranged gives 'ectoral'.
'p'+'ectoral'='PECTORAL'
Can you help me to learn more?
(Other definitions for pectoral that I've seen before include "Ornamental breastplate" , "(Large muscle) of the chest" , "sort of cross" , "Of the chest or breast, like muscles" , "Of the chest, say muscles" .)