Succulent plants are routine in the Channel Islands (5)
I believe the answer is:
cacti
'succulent plants' is the definition.
(I've seen this before)
'routine in the channel islands' is the wordplay.
'routine' becomes 'act' (synonyms).
'in the' indicates putting letters inside.
'channel islands' becomes 'CI'.
'act' inserted inside 'ci' is 'CACTI'.
'are' is the link.
(Other definitions for cacti that I've seen before include "flowers" , "Prickly plants" , "Spiny succulent plants" , "Spiky desert plants in plural" , "Tunas" .)