Prickly plant originally trapping antler, perhaps (5)
I believe the answer is:
thorn
'prickly plant' is the definition.
(I've seen this before)
'originally trapping antler perhaps' is the wordplay.
'originally' says to take the initial letters.
'antler perhaps' becomes 'horn' (antler is a kind of horn).
The first letter of 'trapping' is 't'.
't'+'horn'='THORN'
(Other definitions for thorn that I've seen before include "Letter written long ago" , "irritation when in one's side" , "(Rose's) prickle" , "Prickle on a plant" , "Sharp-pointed tip on a stem" .)