Channel Islands' reservists take on key fortress (7)
I believe the answer is:
citadel
'fortress' is the definition.
(thesaurus)
'channel islands' reservists take on key' is the wordplay.
'channel islands'' becomes 'ci' (this could be a standard abbreviation I've not previously seen).
'reservists' becomes 'ta' (abbreviaton for Territorial Army).
'take on' says to put letters next to each other.
'key' becomes 'del' (del key on a keyboard).
'ci'+'ta'+'del'='CITADEL'
(Other definitions for citadel that I've seen before include "Fortress -- dialect (anag)" , "Fort, fastness" , "Highly-secure building" , "Keep - castle" , "Urban stronghold - dialect (anag)" .)