Desert plants have a role in Channel Islands (5)
I believe the answer is:
cacti
'desert plants' is the definition.
(I've seen this in another clue)
'a role in channel islands' is the wordplay.
'a role' becomes 'act' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'channel islands' becomes 'CI'.
'act' going into 'ci' is 'CACTI'.
'have' acts as a link.
Can you help me to learn more?
(Other definitions for cacti that I've seen before include "Tough plants" , "Prickly types" , "Plants native to arid regions" , "tuna, for example" , "Spiny succulent plants" .)