Islands full of fake plants (5)
I believe the answer is:
cacti
'plants' is the definition.
(cactus is a kind of plant)
'islands full of fake' is the wordplay.
'islands' becomes 'ci' (Channel Islands).
'full of' indicates putting letters inside.
'fake' becomes 'act' (both can mean to simulate or pretend).
'ci' going around 'act' is 'CACTI'.
(Other definitions for cacti that I've seen before include "Spiky desert plants in plural" , "They store water" , "Prickly types" , "Plants of dry regions" , "Spiny desert plants" .)