Prickly plants originally culled before opening of play (5)
I believe the answer is:
cacti
'prickly plants' is the definition.
(plural of cactus)
'originally culled before opening of play' is the wordplay.
'originally' indicates taking the first letters.
'before' says to put letters next to each other.
'opening of play' becomes 'Act I' (first act of a play).
The first letter of 'culled' is 'c'.
'c'+'acti'='CACTI'
(Other definitions for cacti that I've seen before include "they shoot in the desert" , "Plants native to arid regions" , "Spiky desert plants in plural" , "They're decidedly prickly" , "Spiny succulents" .)