Prickly plants play a part in Channel Islands (5)
I believe the answer is:
cacti
'prickly plants' is the definition.
(plural of cactus)
'play a part in channel islands' is the wordplay.
'play a part' becomes 'act' (I've seen this in another clue).
'in' indicates putting letters inside.
'channel islands' becomes 'CI'.
'act' placed into 'ci' is 'CACTI'.
(Other definitions for cacti that I've seen before include "They're decidedly prickly" , "Desert features" , "Tunas" , "they shoot in the desert" , "Prickly plants" .)