Copper tense after punch — that goes against the grain (8)
I believe the answer is:
crosscut
'copper tense after punch that goes against the grain' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'copper tense after punch' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'tense' becomes 't' (abbreviation used in many dictionaries).
'after' says to put letters next to each other.
'punch' becomes 'cross' (I am not sure about this - if you are sure you should believe this answer much more).
'cu'+'t'='cut'
'cut' after 'cross' is 'CROSSCUT'.
Can you help me to learn more?
(Other definitions for crosscut that I've seen before include "It's going against the grain" , "Sawn across the grain" , "To alternate sequences when editing film" .)