Understood rubbish about Channel Islands (5)
I believe the answer is:
tacit
'understood' is the definition.
(I know that understood can be written as tacit)
'rubbish about channel islands' is the wordplay.
'rubbish' becomes 'tat' (low-quality items or material).
'about' indicates putting letters inside.
'channel islands' becomes 'CI'.
'tat' placed around 'ci' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Implied without being said" , "Respect, admiration" , "Attic? (anag)" , "Implied without being put into words" , "without speaking" .)