Plants hint of complexity when play opens (5)
I believe the answer is:
cacti
'plants' is the definition.
(cactus is a kind of plant)
'hint of complexity when play opens' is the wordplay.
'hint of' suggests taking the first letters.
'when play opens' becomes 'acti' (Act One).
The initial letter of 'complexity' is 'c'.
'c'+'acti'='CACTI'
(Other definitions for cacti that I've seen before include "Plants native to arid regions" , "they shoot in the desert" , "Spiny desert plants" , "Prickly Pears perhaps" , "Prickly plants" .)