Hit hard by onset of Gunpowder Plot (5)
I believe the answer is:
graph
'plot' is the definition.
(graphing is a kind of plotting)
'hit hard by onset of gunpowder' is the wordplay.
'hit' becomes 'rap' (rap can mean to knock or hit).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'by' says to put letters next to each other.
'onset of' says to take the initial letters.
The initial letter of 'gunpowder' is 'g'.
'rap'+'h'='raph'
'raph' after 'g' is 'GRAPH'.
(Other definitions for graph that I've seen before include "Mathematical diagram" , "Diagram representing relationships between quantities" , "Diagram showing relationships between things" , "Visual representation" , "Symbolic diagram" .)