Boffin in tests spread across Channel Islands (9)
I believe the answer is:
scientist
'boffin' is the definition.
(I've seen this before)
'in tests spread across channel islands' is the wordplay.
'spread' indicates an anagram.
'across' indicates putting letters inside.
'channel islands' becomes 'ci'.
'in'+'tests'='intests'
'intests' anagrammed gives 'sentist'.
'sentist' placed around 'ci' is 'SCIENTIST'.
(Other definitions for scientist that I've seen before include "entomologist maybe" , "Is it insects, strangely, the learned person knows about?" , "It's nicest for one engaged in specialist studies" , "Researcher - boffin" , "`Tis nicest kind of specialist" .)