Trunk for captain going round Channel Islands (9)
I believe the answer is:
proboscis
'trunk' is the definition.
(synonyms)
'for captain going round channel islands' is the wordplay.
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'captain' becomes 'boss'.
'going round' is an insertion indicator (some letters go around others).
'channel islands' becomes 'CI'.
'boss' enclosing 'ci' is 'boscis'.
'pro'+'boscis'='PROBOSCIS'
(Other definitions for proboscis that I've seen before include "Feeding appendage of elephant, insect" , "Snout" , "Elephant's trunk" , "Elephantine nose" , "MONKEY" .)