Landowner endlessly in retreat (4)
I believe the answer is:
lair
'retreat' is the definition.
(both can mean a private hideout)
'landowner endlessly' is the wordplay.
'landowner' becomes 'laird' (Scottish term for a landowner).
'endlessly' means to remove the last letter.
'laird' with its final letter removed is 'LAIR'.
'in' acts as a link.
(Other definitions for lair that I've seen before include "animal refuge" , "A nautical hail" , "Place of seclusion (informal)" , "office?" , "Hideaway, den" .)