Spiny plants originally cleared before start of play (5)
I believe the answer is:
cacti
'spiny plants' is the definition.
(I've seen this in another clue)
'originally cleared before start of play' is the wordplay.
'originally' indicates taking the first letters.
'before' means one lot of letters go next to another.
'start of play' becomes 'act i' (Act I).
The initial letter of 'cleared' is 'c'.
'c'+'acti'='CACTI'
(Other definitions for cacti that I've seen before include "growing group" , "Tunas" , "Spiny succulents" , "Prickly Pears perhaps" , "flowers" .)