Castle requiring a single key (7)
I believe the answer is:
arundel
'castle' is the definition.
(I've seen this before)
'a single key' is the wordplay.
'single' becomes 'run' (cricket).
'key' becomes 'del' (del key on a keyboard).
'a'+'run'+'del'='ARUNDEL'
'requiring' acts as a link.
(Other definitions for arundel that I've seen before include "Castle in W. Sussex" , "West Sussex town with a castle" , "Norfolk seat" , "English public school" , "somewhere in W Sussex" .)