Ripped when pierced by hard prickle (5)
I believe the answer is:
thorn
'prickle' is the definition.
(I know that prickle can be written as thorn)
'ripped when pierced by hard' is the wordplay.
'ripped when' becomes 'torn' (synonyms. I am not sure about the 'when' bit.).
'pierced by' indicates putting letters inside (some letters pierce a hole in others).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'torn' going around 'h' is 'THORN'.
Can you help me to learn more?
(Other definitions for thorn that I've seen before include "irritation when in one's side" , "eg Prickle on rose" , "North (anag) -- point" , "Spiny bush" , "Spike on a rose bush" .)