English castle with a Lord’s single key (7)
I believe the answer is:
arundel
'english castle' is the definition.
(castle in Sussex)
'with a lord's single key' is the wordplay.
'with' is a charade indicator (letters next to each other).
'lord's single' becomes 'run' (one run in cricket - eg at Lord's).
'key' becomes 'del' (del key on a keyboard).
'a' put next to 'run' is 'arun'.
'arun'+'del'='ARUNDEL'
(Other definitions for arundel that I've seen before include "West Sussex town with a castle" , "Norfolk seat" , "English public school" , "Castle in W. Sussex" , "somewhere in W Sussex" .)