Step outside a lake castle (6)
I believe the answer is:
palace
'castle' is the definition.
(synonyms)
'step outside a lake' is the wordplay.
'step' becomes 'pace' (pace is a kind of step).
'outside' indicates putting letters inside.
'lake' becomes 'l' (geographical abbreviation).
'a'+'l'='al'
'pace' enclosing 'al' is 'PALACE'.
(Other definitions for palace that I've seen before include "Castle" , "Royal dwelling" , "Royal residence" , "Grand building" , "football team" .)