Huge canoe at sea — Channel Islands coming up (7)
I believe the answer is:
oceanic
'huge' is the definition.
(I've seen this before)
'canoe at sea channel islands coming up' is the wordplay.
'at sea' indicates an anagram.
'channel islands' becomes 'CI'.
'coming up' is a reversal indicator.
'ci' in reverse letter order is 'ic'.
'canoe' anagrammed gives 'ocean'.
'ocean'+'ic'='OCEANIC'
(Other definitions for oceanic that I've seen before include "vast waves?" , "Relating to large areas of water" , "Of the open sea" , "Cocaine (anag)" , "In deep water" .)