Merlin working to support King in fortress (7)
I believe the answer is:
kremlin
'fortress' is the definition.
(I've seen this before)
'merlin working to support king' is the wordplay.
'working' indicates anagramming the letters.
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'king' becomes 'K' (abbreviation for king in chess).
'merlin' anagrammed gives 'remlin'.
'remlin' after 'k' is 'KREMLIN'.
'in' is the link.
(Other definitions for kremlin that I've seen before include "House of the Russian government" , "Citadel of a Russian town" , "Seat of Russia's government" , "Castle" , "powerful red element?" .)