Plants contributing to atmospheric activity (5)
I believe the answer is:
cacti
'plants' is the definition.
(cactus is a kind of plant)
'contributing to atmospheric activity' is the wordplay.
'contributing to' indicates a hidden word.
'CACTI' can be found hidden inside 'atmospheric activity'.
(Other definitions for cacti that I've seen before include "Tough plants" , "Prickly types" , "they shoot in the desert" , "Prickly Pears perhaps" , "Spiny desert plants" .)